What Causes Excessive Vaginal Lubrication To Inhibit Intercourse?

My boyfriend told me that for the past three months, my natural lubrication during sex is causing his penis to become numb and he gets a burning sensation. He went as far as getting up in the middle of sex to wash it off so he could come back and gain back sensation. He thought it was something wrong with him and went to the doctors to get tested for anything that might be wrong but nothing came up. I have had no other sexual partners and am clean for STD's. Whats going on?