thrush

I had thrush in my mouth last September and I treated it with Nistatin. Three weeks later I had thrush again and I treated it with Nistatin again ( 5 days three times a day). Then, I did the ELISA test and the result was NEGATIVE for HIV. Now, in november, I had thrush again. Now I am taking NISORAL for fifteen days one medicine a day. I don't know what is causing it and I am worried about it. My question: is it possible to have HIV,  to get a negative result in ELISA test and at the same time to have thrush in the mouth?  I am a homossexual male, 33 years old and I have done oral sex without condom and without ejaculation in the last six months.