Welcome to the HIV forum.
Oral-anal contact is generally believed to carry little of any risk for HIV transmission, in either direction -- but there are no data and there might be some small risk. (Masturbating with semen as lubricant definitely carries no risk.) Most likely your "flu-like" symptoms are due to something other than HIV, maybe even influenza itself -- but you don't provide enough detail about them for me to know. However, the smart thing is for you to see a health care provider knowledgeable about HIV and let him or her advise you.
Now here comes the lecture: It is really, really dumb for any male to have sex with another male without both men knowing and sharing their HIV status. In other words, you should have asked before you got started, and avoided sex entirely if he said he was positive, didn't know, or seemed evasive in his answer. Even if oral-anal sex is low risk, sometimes things get out of hand in the heat of the moment. Knowing and sharing HIV status is just as important for safe sex between men as condoms for anal sex. Please start having such conversations routinely before having sex. Otherwise it's a fair bet HIV is in your future.
Good luck-- HHH, MD