White coating on the tongue is very nonspecific; it can be normal or can appear in response to colds or other common viruses--and it can appear in response to antibiotics, including metronidazole (Flagyl). Thrush always involve more than the tongue; I doubt you have it. As to whether you have HIV, you don't provide enough information. It is extremely rare to acquire HIV after a single episode of unprotected sex. If you are an injection drug user, a gay man, or a sex partner of one of those, then you should get HIV tested from time to time. Otherwise, don't lose any sleep over this episode.
By the way, why were you prescribed Flagyl?
Good luck-- HHH, MD
I had a parasitic infection. by the way, I am gay. thanks for the info.