Help! Braxton Hicks?

For the past two days I've been having some belly pain. I figured it was round ligament pain, which I had way early in my pregnancy, but now I'm not so sure. I noticed while k was in bed last night that I think my uterus is tightening during some of these cramps. They definitely hurt! I mostly get them when I get up from sitting for a long time or when I roll over in bed. I'm 21 weeks right now. Any thoughts? Should I call the midwife today?