Braxton hicks

First time mom here and I want to know how to tell the difference between baby rolling around, braxton hicks, and real contractions. At 31 weeks I went in to be monitored because I was not feeling baby move as much as normal. Everything turned out fine but with the monitor they could see I was having some contractions. I did not feel anything though so I was a little confused. Now I am 33 weeks and have recently felt some tightening but its mostly on one side and I have a hard time deciding if it is baby pushing on me or braxton hicks. How do you know?