Hi Lucy,
If you have only been sleeping on your new mattress for 10 days then it may be a good idea to give things just a little bit more time because there will be a break in and adjustment period over the first few weeks with any new mattress which may make a difference (see post #3 here)
While it’s not really possible to “diagnose” mattress comfort issues on a forum with any certainty because there are too many unique unknowns and variables involved that can affect how each person sleeps on a mattress in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) or any “symptoms” they experience … there is more about the most common symptoms that people may experience when they sleep on a mattress and the most likely (although not the only) reasons for them in post #2 here.
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
These posts are the “tools” that can help with the analysis, detective work, or trial and error that may be necessary to help you learn your body’s language and “translate” what your body is trying to tell you so you can make the types of changes that have the best chance of reducing or eliminating any “symptoms” you are experiencing.
Upper body or neck symptoms can often be the result of a pillow issue because if you change your mattress for a different mattress that allows your upper body and shoulders to sink in either more or less than your previous mattress then the distance between your head and neck and the mattress surface can change and may require a thicker or thinner pillow to keep your head and neck in good alignment.
If the mattress itself is too soft then some of the suggestions in post #4 here may also be helpful and in particular I would make sure that the mattress is on a suitable support surface that doesn’t have any flex under the mattress. You can check this by putting the mattress on the floor and seeing if it makes a difference.
The best use for a topper is to provide some additional softness to a mattress that is too firm and it’s much more difficult to firm up a mattress that is too soft because this would normally require identifying which layers in your mattress are too soft and then removing and replacing either the comfort layers that are too thick and/or soft or a support core that is too soft. If you add a firmer topper to a mattress that is too soft then it may be too firm for you to be comfortable for you to sleep on and the topper will usually still “bend” or compress into the softer layers underneath it so you may end up with a temporary or partial solution or at worst you can make the problem worse if the topper you add is soft enough to be comfortable to sleep on.
If the memory foam layers in your mattress are more temperature sensitive then adding a couple of inches of latex in a firmness level that is comfortable for you to sleep on can sometimes firm up the memory foam underneath it enough to make a difference or at least lengthen the time that the memory foam takes to soften over the course of the night but the only way to know for certain whether this will make “enough” of a difference to help with your lower back pain would be based on your actual sleeping experience so the return policy for the topper would also become a more important part of the “value” of your purchase just in case it doesn’t work out as well as you hope for.
Phoenix