Hard uterus & not sure what to do!

Hey ladies! I’m 16 weeks pregnant and starting to get a little concerned. Last night my boyfriend and I had sex and like all other times my uterus cramped up (even though I didn’t orgasm) and got really hard. Well normally by the end of the night it goes back to normal but this morning when I woke up it was still just as hard and has stayed this way all day. I’m not sure if this is normal since this is my first pregnancy, can anyone help me out?