Aiyoh, that hardening feeling is super common in second trimester, especially around your weeks, a few of us went through the same thing and it can feel quite uncomfortable, like constipation or needing to poop but nothing comes out. Since you're first time pregnant, it's good to check with your doctor just to make sure everything's okay, but from what I've seen, it's usually Braxton Hicks (practice contractions) mixed with the baby taking up space and sometimes constipation from pregnancy hormones. Try drinking more water, eat more fiber, and maybe gentle walks can help, one of my mum friends swears it settled down once she got a routine going.