Contraction question..?

Tabitha • 24. FTM! Due on Sept. 29, 2015 with a baby girl💜
I was wondering if any mamas out there could try to explain what contractions feel like. Everyone says your stomach hardens and it's like period cramps...but the pain, it like a stinging or burning for like 30 seconds? I am a first time mom and in the middle of the night I had a sharp stinging pain in my tummy when I laid a certain way (after peeing). I didn't think anything of it until now as I went to sleep after the pain was gone (lasted about 20 seconds). My doctor is closed until 8 tomorrow morning and wanted to reach out to the glow community. I am 33 weeks today!