stomach pains

I keep waking up with this real bad stomach pain, it makes me feel like I have to poop, but when I go to the bathroom, I don't poop... I only really feel it when my stomach tightens. Could this be my contractions getting stronger? Or just a stomach ache?
Edit: I'm 37 weeks and 1 day and I think I should add I only feel this after waking up. I can feel that my stomach is tight and it hurts like a stomach ache, but then it eventually goes away.