Hi vrenlos,
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.
It may also be a good idea to check the support system under your mattress to make sure that it is perfectly flat and doesn’t flex or bend under the weight of your mattress and the people sleeping on it. You can check this by putting your mattress on the floor to see if it makes any difference (a suitable support system should have very little to no flex and would be very similar to the floor).
It’s also possible that you could have a pillow issue rather than a mattress issue and it’s not uncommon that a new mattress will also need a new pillow with a different thickness to keep your head and neck in good alignment.
Phoenix