Painful bloating and cramping at 16wks...is this normal?

Christyn
I'm 16 weeks pregnant with my second. Recently, and with increasing frequency, my whole belly puffs out painfully....like can't stand up straight painfully. My belly really rounds out and becomes taut and cramps from the bottom of my ribs to my pelvis. I've been a bit constipated and have been taking a fiber vitamin but otherwise all is normal. This didn't happen during my first pregnancy. Has anyone else experienced this? What do I do?