Hi shal!
Were you diagnosed by a doctor and told that it was oral thrush?
I've had a similar situation, I have a very mild white coating on the back of my tongue, it started when I took an antibiotic that I was allergic to, it never really disappeared since that time, it did worry me for a while, and because of my constant worry of what this might be, it started to get worse until somehow I have gotten over it.
Anyway, if you smoke, take drugs or even drink too much alcohol, a whitecoating would develop on your tongue, that's what my doctor had told me.
Stress also plays a major role in creating such symptoms, I've never gotten involved into a real risky behavior, however when I started reading about the HIV and it's symptoms online, I started getting every single symptom out there.
Goodluck with your results, I am pretty sure you're negative.