Preterm Labor?

Marah
So I'm just over 31 weeks (baby is measuring more at 33 weeks, but haven't confirmed due date change) 
Anyway, yesterday my back started hurting. No biggie. Went about my day and it continued to hurt. After my son's baseball game my back was on fire and then I had an upset stomach. Woke up this morning with more intense back pain and diarrhea (sorry tmi). I've been laying on my left side and drinking water and nothing is helping. My back feels like its on fire. I'm getting pains down the side of my stomach and in my lower pelvic area. Thankfully baby is moving at a decent rate. 
I honestly don't remember what contractions or labor feel like. My oldest is 7. 
What do you ladies think?