how to tell if it's contractions?

I'm almost 39 weeks. I keep feeling like all my stomach muscles are straining towards my belly button, I dunno how else to describe it. It feels very tight, and a little hard to breathe through. It lasts for only a minute or two, then it goes away. Thr last few days I'd have one and an hour later another one, then they stop. Lately I get them every half hour, every twenty minutes, now it's to every fifteen minutes. I just don't know cause it's not super painful.yet.