My belly gets rock hard when working out... is that normal?

My doctor said I can workout as long as it’s not causing me pain. Recently my belly gets super hard when working out but there’s no pain. I always end up stopping but after speaking with my doctor about it she says she’s ok with me continuing as long as I’m not in pain, cramping or bleeding. I just never experienced my belly getting rock hard while working out in any of my pregnancies before. Has anyone experienced this before?

*Edit*

I am 20 weeks pregnant. When the rock hard tightness starts it usually stays even about 15-20 mins after I stop working out but during the workout it’s there consistently; it doesn’t come and go.