per se does not cause anal fissures. There is no such study that proves the same. Let me explain it to you in a simple way. HIV is common in homosexual men who indulge in URPAI (Unprotected Receptive or Penetrative Anal Intercourse). Since the anal mucosa is very thin and fragile, it tends to break causing anal fissures. In the presence of anal fissures, the risk of transmission of HIV is higher.
This is how HIV and anal fissures are related. I have not come across anywhere about literature saying HIV causes fissures.
If you have had the tests correctly and repeatedly i.e. 2 tests 6 months apart saying that you are HIV negative, then you probably are HIV negative.
Hope this helps you.
Wish you good health.