Discharge, Cramps - Third Trimester

okay so today I had my regular check up that I go to every two weeks. I am one week away from the third trimester. This whole week I've been having cramps and when I was at the checkup the nurse said to go into the hospital if I have cramps or discharge. My partner and I went to Walmart to pick up a changing pad and I got this huge cramp and then a gush of watery fluid. we immediately went to the hospital and they checked for amniotic fluid and monitored my 'contractions'. after a while they said they didn't know what's wrong and sent me home. The watery fluid had a pink tone in it and they said I didn't have an std, UTI, Sti, etc. what could this be??