What Could Be The Reason For Having Numbness In Legs And Vaginal Burning After Ejaculation?

My bf and I have both been tested for std and we are completely fine. But I find that certain times when he ejaculates in me, my vagina burns very bad and my legs go numb. Sometimes it is tolerable while other times its unbearable. When I try to pee after sex, I can barely let anything out because it burns so bad. I know it is not poor lubrication because I get very wet and it has never been a problem. Like I said, there are different levels of burning depending on the day. Some days are worse than others. We have tried condoms but they tend to to irritated me as well. I am on birth control. Any ideas on what the the problem could be??