Mattress and back pain

Hello, we purchased a new mattress about a month ago, or a little bit over a month. We decided to go with a firm mattress. I should of done a bit more research but i did not.. in the beginning it took some time to adjust maybe a week i had back pain, which i read online its normal for the back to adjust. After that, it went away... no issues. Then 2 days ago, i started having back back again. Is the mattress i purchased not for me? why the back pain keeps coming back?