Hi Dazra,
I would certainly echo the comments that MaryPT made about memory foam. I should also add that it not only changes with temperature but also with humidity and it also has a property called “creep” which means that like all viscoelastic materials it will “relax” under constant pressure and become softer just with time alone regardless of any changes in temperature or humidity.
I should also mention though that there could also be other issues involved and I would not take chest pains lightly. I would make a point of seeing your primary health care provider to make sure there isn’t any hidden causes for your pain that you may not be aware of.
You also mentioned that you weren’t “tossing and turning” over the course of the night which is good if you tend towards moving too much because of pressure or alignment issues but I should also mention that one of the drawbacks of memory foam in some cases (depending on the type of memory foam and the thickness and softness of the memory foam layers) is that it can restrict free movement on a mattress and part of healthy sleeping involves the ability to easily change position on a mattress over the course of the night to relieve pressure and maintain good circulation in various parts of the body. Without this, muscles, ligaments, tendons, and joints can stiffen or circulation can be impaired without any motion which can also cause pain and stiffness in the morning. This includes the tissues of the rib cage which are also subject to pressure.
As MaryPT also mentioned the mattress may not be suitable for you in terms of keeping you in good alignment in all your sleeping positions over the course of the night in which case your muscles will be doing the work of trying to keep your spine and joints in neutral alignment instead of the mattress. This will also affect the ability of your spine to decompress and the discs in your spine to rehydrate and regain their cushioning ability over the course of the night.
Finally … your mattress has 5" of memory foam which would be a risky construction for many people. Not only does a mattress need to keep you in good alignment along your spine but it also needs to keep you in good alignment from side to side and if you are sleeping in a “hunched” position (with your arm “out there”) or if you are twisting while you sleep then this could also cause issues and pain and discomfort in the morning. Some sleeping positions can also impair good breathing by restricting the chest cavity.
One final comment is that pressure issues are often what you feel when you are going to sleep at night but alignment issues are normally what you feel when you wake up in the morning either with or without back pain or muscle stiffness and soreness. While it’s not possible to “diagnose” the specifics of what may be causing your pain … I think the odds are good that it is an alignment issue of some type which is compressing or causing stress in the area of our pain.
A mattress has two main functions which is to provide good pressure relief and to provide neutral alignment over the course of the night in all your sleeping positions. Beyond this it needs to be “safe” in terms of materials and also to provide a good microclimate in terms of temperature and humidity control close to the body. Everything after that is preferences.
It’s true that a mattress will go through an initial “break in” period where the foam will lose some of its “false loft” and soften and the cover will loosen and become more pliant. You will also go through an adjustment period with any new sleeping surface … especially if it is very different from what you were used to and the body needs time to “unlearn” it’s old sleeping posture and get used to a new one. Both of these normally take a few weeks (usually less than 30 days) and it can be important to give any new mattress enough time that you can be reasonably sure that what you are feeling is a good indication of your longer term experience as well.
Having said that … your “symptoms” are not typical of an adjustment period and while I am very hesitant to guess because I know nothing about your body type or any other relevant information about you … I would think that the odds are good that your mattress may already be too thick and soft in the comfort layers and that you may need to make a firmer choice. If you do choose to exchange your mattress … then post #1 here includes some links to guidelines for testing for pressure relief and alignment more carefully and objectively so you can avoid the “temptation” to choose a mattress based on the subjective perceptions of “showroom comfort” and “feel” alone which can lead to issues in your “real life” sleeping experiences.
Your job is to determine as carefully as possible whether your issues are pressure related or an alignment issue (which is much more likely) and then with careful and objective testing to choose a mattress that is more suitable for your needs if it turns out that what you have isn’t working for you.
@MaryPT,
Thank you so much for taking the time to post the comments you did. It’s a refreshing change to see a health professional taking a serious interest in mattress construction and materials and their effect on the many health issues that you deal with on a daily basis. I see so many uninformed comments and in particular many “referrals” towards memory foam mattresses from many health professionals in the mistaken belief that somehow they are intrinsically “better for back issues” that are not based on any real knowledge and information or don’t take into account some of the risks that memory foam can involve in some types of constructions (particularly thicker layers).
While I may not necessarily agree with innersprings being the only suitable support layer … I think we share a common belief that good alignment over the course of the night is an essential aspect of healthy and regenerative sleep.
Thanks again for taking the time to post your insights and comments … I appreciate it 
Phoenix