Doing heroin can for sure compromise your immune system, for a whole variety of reasons (the actual heroin itself is, surprisingly, not really one of them), so if you've come down with some ailment that's associated with a compromised immune system, it's not terribly surprising. It's also true that a fair number of people who share needles contract HIV. And HIV, in turn, of course, leads to AIDS, which is by nature a disease that consists of a compromised immune system. I'm not a doctor, but I would guess based on what I know about HIV/AIDS, if you tested negative for HIV a month ago, there's no way that what is wrong with you now is related in any way to HIV, because it takes many months, typically years, before HIV graduates into actual AIDS, and during that time, your immune system works more or less normally. I'd say it's far more likely you just have a weakened immune system from shooting dope all the time ... and HIV/AIDS is not part of the picture in any way.
What did the doc who diagnosed you with this disease say about the situation? Does he know you're a heroin user, or not?
What is it, toxoplasmosis or whatever it's called?