Painful (lower part of the opening)?

My boyfriend and I have been having for a month now and it's been fine until the past 3 or so days. The lower part of the opening (towards the a$s) becomes really painful after . It burns when I put lubricant on it and entry hurts too much when we want to go again. I'd liken it to a carpet burn, so could it be coz of the position of the during (i.e. legs up on his shoulders, laying on my back and him going in and out horizontally, rubbing that area) ? Its really ruining it so what should I do ? I really don't want to go to a doctor though.