Every time i have sex with my girl friend she says my sperm burns her what could be wrong?

Lack of a condom: Unless you are seeking parenthood, for starters, the absence of a condom. Would resolve question. Could be pH difference (hersyours), might be a sensitivity or allergy to semen component. Might be an indication of an infection. Would suggest exam and testing, then condom use.

Try a condom: sperm allergies are uncommon, but possible. they can cause severe pain during/after intercourse. using a condom (which would prevent sperm from touching her vagina) would be one informal way to test this. other infections or dysfunction should be ruled out however -- potential problems could be urinary tract infection, sexually transmitted disease, psychological discomfort with intercourse

Lots of options: The sensation of semen burning can be due to a number of things. Semen can cause a burning sensation if the vagina is inflamed for any reason - mechanical irritation, vaginitis, or changes in vaginal chemistry. Additionally, there is a human seminal plasma protein hypersensitivity which is like an allergy to semen. Please have her see her doctor for a discussion and evaluation.