Dr's preg test was + then turned -???

I've had two +hpt (taken with FMU) and today had my doctor's appt. I gave a urine sample and the dr came in and told us we were pregnant! I have been having some pretty severe bloating so I also had an u/s to see if I have OHSS. Abdominal cavity was FULL of fluids, and the nurse confirmed I was hyper stimulated. She also said my pregnancy test had turned negative. Has this happened to anyone else?? She said the fluids from my OHSS could be diluting the hCG in my system and ordered blood work. Talk about an emotional roller coaster - my pregnancy was confirmed for a whole 30 minutes. My husband left before the u/s as he had an appt to get to so I had to text him that we actually might not be pregnant. What the heck?!