How do you know if you're having them?

I'm a ftm, about 22 weeks and I keep feeling a tightening in my lower abdomen (uterus). It's not horrible but maybe a tad uncomfortable at times. I thought maybe I had to go to the bathroom, but it didn't really help. I just got back from vacation and I didn't over do anything. Is is braxton Hicks contractions? What should I do? Call the doc? ER?