Hi Katia,
Congratulations on your new mattress 
You certainly made a high quality choice but of course regardless of the quality or durability of your mattress how well you sleep on it is always the most important part of “value”.
Your mattress certainly seems to be “in the range” that would work well for many back sleepers with your weight and body type but as you mentioned your health issues can also significantly complicate the choice of which mattress works best for you and you may be well outside the “averages” that would work well for others with a similar body type or sleeping style. Since I can’t feel what you feel or see you sleeping on the mattress there really isn’t any way for me to know or “diagnose” how much of your lower back issues are coming from your back condition or how much they are a result of your mattress or the type of changes in your mattress that may help you (to the degree that any mattress is able to help).
Having said that … there will be a break in and adjustment period with any new mattress over the course of the first few weeks and you have only been sleeping on your mattress for a week which really isn’t long enough to consider making any changes yet (see post #3 here) so my first and “best” suggestion for the moment would be to spend a few more weeks on your mattress (30 days is usually a good idea) before deciding whether you need to make any changes or assessing your situation to help you decide on the type of changes to make.
If after about 30 days or so you are still experiencing “symptoms” on your mattress and you haven’t noticed any changes in your sleeping experience then it would be a much better time to start the “analysis” or “detective work” and possibly even some trial and error that may be required to identify the type of changes have the highest odds of success.
When the time comes I can certainly link you to some more detailed information that may be helpful and which in combination with a conversation with Alan and possibly some additional testing on other mattresses may help you decide on the next step but for now the best approach is probably to do nothing and give your mattress more time to break in and for your body to get used to a sleeping surface that is very different from what it is used to. Based on your comments about your previous mattress your body may have adapted to a “learned alignment” from your previous sagging mattress and it can take some time and you may experience some “symptoms” while your body is getting used to sleeping in better alignment.
Sleeping with a pillow under your knees can certainly help to flatten out the lower back and help with lower back alignment … especially if there are medical issues involved … and since this seems to be helping I would continue with this as well.
If it’s one of the thinner and more stretchy “thin membrane” type of protectors and it isn’t too tight and creating a “drum effect” on your mattress then it’s unlikely to be the reason for your symptoms but the only way to know for certain would be to try sleeping without it for a couple of nights to see if it makes any difference.
While the most common (but not the only) reason for lower back issues is a mattress that is too soft … I would avoid speculating about any of this until you have spent more time on the mattress to see how your experience changes over the course of the next few weeks.
Phoenix