Hi fridadc.
I‘m sorry you’re having some trouble with your shoulder/neck adjusting to your new mattress but I am glad that as far as your alignment the mattress it is working well. Unfortunately I can’t pinpoint or diagnose via an online forum the reasons that you might be experiencing this pain, but I can make some general comments about things to look into or adjustments may be helpful to ease shoulder pressure for side sleepers in the hope that they are useful to you.
First off, I’d make sure that you reevaluate your pillow in combination with your new mattress to make sure that it is providing a decent alignment to keep your cervical/upper thoracic region in a relatively neutral arrangement. Especially with the body adjustments that come with a new mattress the old pillow might have an improper thickness which is a common cause for shoulder issues. Using a body pillow can be a good thing and resting your free arm upon that can be helpful. Placing a pillow behind you to slightly lean against can also take a bit of stress off of your shoulder joint area.
Next, you may wish to reevaluate how you are sleeping upon your side and the location of your arms, as this can also make a large difference in shoulder pain, regardless of mattress types. While on your side, fold your arms across your chest (like what is commonly depicted on an Egyptian sarcophagus). So if sleeping upon your right side, your right arm would be crossed over your chest, with the elbow pointing down toward your waist area and your hand closer to your left shoulder. Doing so adbucts your shoulder girdle and takes some of the pressure off of your shoulder joint. Also, consider placing a thicker pillow in front of you and place your free arm (in his case, the left arm) a top this pillow to help support it in a more neutral position and take pressure off of both shoulders.
Some extra surface plushness by adding a topper can be an option. Since you don’t like the feel of either latex or Memory Foam that leaves you with just the option of a upper layer of polyfoam or natural or synthetic fibers or materials like down.
There is more information about choosing a topper and a link to the better online sources I’m aware of in post #2 hereand the topper guidelines it links to which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. A good exchange/return policy can also reduce the risk of an online topper purchase so I would make sure you are comfortable with the options you have available after a purchase just in case the topper you choose doesn’t turn out as well as you hoped for.
The links contained in post #11 here provide quite a bit of good information about looking into possible reasons for should joint pain.
There is also more information in post #2 here that talks about the various symptoms that people may experience on a mattress and some of the reasons that can cause them that can help with the detective work or trial and error that may be necessary to identify the types of changes that may be helpful.
Shoulder and arm issues can come from many sources, such as a mattress that is too firm and puts direct pressure on the shoulders, the shoulder blades, or on the back muscles and can also cause soreness or numbness and tingling in the arms. It can often come from postural issues. If the mattress is too firm, then if you sleep on your side your shoulders may not sink in enough to relieve pressure and your upper body can “twist” away from the pressure so your upper body is “twisted” more forward while the lower body is still on its side. This spinal twisting or torsion can twist the spine in the upper body and lead to soreness in the area of the twist. There is some much more detailed information, including some sleeping postural tips, on shoulder and arm issues in posts #2 and #3 here .
Shoulder issues can also come from sleeping in the same position for a longer period of time than normal. If you are sleeping deeper/longer on your new mattress, you may be repositioning less often, and this could be causing your shoulder issues, as you may be staying in the same position longer than your body is used to. I can’t speak to why you’re suddenly experiencing a different sensation. Usually when that happens it points to something changing in your flexibility, strength or fatigue. A new mattress will soften slightly as it breaks in, but there wouldn’t be as much of this with latex as there would be with polyfoam, and you’ve stated that your alignment is still good (which sleep ergonomic research would prioritize over surface plushness).
Phoenix