Hi sweetredhead,
As you probably know it can take some time for a new mattress to break in and for your body to adjust to adjust to a new sleeping surface that is different from what it’s used to (see post #3 here)
While there are many reasons that can cause a pinched nerve that may not be related to your mattress … it’s also not uncommon at all that you may need a different pillow when you buy a new mattress because you may be sinking into the mattress more or less than your old one so the gap between your head and the mattress surface may be different now than it was before so even if your pillow was brand new it’s very possible that you may need a different pillow to keep your head and neck in neutral alignment.
There is more information about the most common symptoms that people may experience on a mattress and some of the most likely (although not the only) reasons for them in post #2 here that may also be helpful.
Phoenix