Received oral sex that has led to prostatitis. all std negative including m gen. what could have caused it if not by std? am i contagious?

Mouth bacteria: The oral cavity has one of the one of the greatest number of disease producing bacteria(pathogenic) of anywhere outside of the anal canal. Yes, it is even "dirtier" then the vagina That is why a human bite is especially infective once it breaks the skin. Listerine swigs prior to oral sex have been known to minimize the number of oral bacteria prior to engaging in sexual activity of the oral kind.

Pelvic pain?: Prostatitis is not due to any STD, and oral sex is not known to cause prostatitis. Those events probably are unrelated. Or maybe you have chronic pelvic pain syndrome, which causes symptoms similar to prostatitis; read the very good Wikipedia article. CPPS is uncomfortable but harmless. Your local NHS GUM clinic would be an excellent option for expert care for it. Antibiotics probably won't help.