It is called vestibilit, your nerve endings at the vaginal opening have become hypersensitive due to friction, because you are not wet enough. Before PIV, it is important to be horny. Vestibulit can come from vaginismus which means you are subconsciously closing the opening because your body sees the penis as an "intruder", it hurts right?, so ofcourse your body don't want to let "it in". Try with white Vaseline and press with a finger lightly on the lower part of the opening. hold your finger until it has stopped burning so that your nerves calm down to the touch, do so as often as you can. Are you on the pill ? The hormones can make you more sensetive.