31 weeks 1 day and Worried

I'm pregnant with baby number 2, and I'm 31 weeks and 1 day. Ever since yesterday my belly has tightened up and will not go back to normal. If I lay flat or move a certain way, it hurts right under my belly. I don't know what to do, as this didn't happen with my first. She's moving less, but definitely still moving. I've also had back pain on and off. Should I go to the hospital? Or is this normal?