What Causes Severe Irritation In The Vagina Post Sex?

Im 26 years old and every time my husdand and I have sex and he has a orgasm in side of me it burns Ive gone to the doc several of times and theres no infection or std's. I burn so bad to the point I cant urinate I have to put a piece of ice in a wash towel. mMy husband and I have had many different cultures done but everthing comes back negative. This was going on before I was with my husband so I know its not him its me. Doctors cant seem to find the problem I get my paps every year. I have tried different yeast infection meds and bacteria meds.