31 weeks 3 days, woke up to bad pain?

ok so i woke up at about 330 almost 4 am and i was having a kind of sharp pain in my lower abdomen, and i had to go to the bathroom really bad(just pee so it wasn't gas or anything)so i got up and went and the pain went away a little, i went and got back into bed, first slightly on my back but the pain came back stronger, so i turned to my left side and it still hurt, not as bad but still very painfull, so i sat up, and the pain disappeared, so i ended up sleeping in an almost sitting position. any idea what this pain was? this is my first pregnancy, so it scared me a lot, but because the pain went away when i sat up i didn't think it was anything to bad, because ive read that if the pain doesn't go away when changing positions, then i need to call a doctor or something.