Hi girl_toronto,
I switched your post to your own topic so it wouldn’t get mixed in with a more general topic.
It’s certainly possible that your mattress is too soft which would be a common cause of lower back pain that goes away after you get up but of course I can’t know for certain.
Only you can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved for anyone to be able to suggest a specific mattress based on specs (either yours or a mattress), health conditions, individual circumstances, or “theory at a distance” (see mattress firmness/comfort levels in post #2 here). Your best chance of success in choosing the most suitable mattress will be to use the testing guidelines in step 4 of the mattress shopping tutorial.
Unfortunately, and like many of the members here that have been in a similar situation, you are in a somewhat difficult position where you need to exchange a mattress at a store that I would normally suggest avoiding and where there may not be any particularly good quality/value options available to you.
There are some suggestions and ideas in post #2 here about the two main strategies that you can use that can help you make the best of a difficult situation. There is also more in post #13 here about the most important parts of the “value” of a mattress purchase or exchange that may be helpful as well.
Phoenix