What causes HIV false positives? ?

I've been donating blood for a while and I got a letter saying the general test said positive. But further testing proved negative. So I figured nothing was wrong. And today when I went in to give she said I wasn't eligible, but the computer didnt say why. So I'm guessing it had something to do with that letter. It is virtually impossible for me to have it (I'm a virgin. And neither parents have it). And the letter even said I was negative. So now I have to go to the doctor for a test. Bring a note to them. And let them retest my blood. I'm just wondering what would cause the false positive.