Contractions??

I am 28 weeks exactly and for the last 2 hours I have been having menstrual like cramping in my back, towards the left side right above my back. It almost feels like I have to poo (tmi) but everytime I try to, nothing comes. I had my daughter 2 years ago so of course I don't remember what labor was like with her, and I never experienced Braxton Hicks or false labor contractions with her either. I have had the Braxton Hicks all day as well. Could I be constipated or should I be a little more concerned?