Back pain. Someone please have an answer.

Abigail • Pregnant with #2 & twins this time!
I'm 35 weeks with fraternal twins. I was standing in the kitchen, doing absolutely nothing and I got a constant, sharp, stabbing pain in my upper middle back. It started to subside a little, but now I'm also getting contractions and trying to time them. Along with my contractions, I get a very slight period like pain. Anyone know what's going on? I don't want to go to the hospital if it's nothing.