I sometimes get the same type of pain when I am having sex with my husband...I don't have anal sex though..I wasn't sure if you were talking about that or doggystyle..But when we do it doggystyle I seem to get worked up more quickly than anything and the more and more turned on I would get my stomach would start hurting so I would just end up going to missionary and it would still be there but not as painful..Afterwards try taking a warm bath. Also before sex try not and drink anything with caffeine in it..I found that everytime I drank alot of pepsi beforehand the pains would start...I really don't think anything is majorly wrong with you...Sex is sometimes painful for every woman..It could even be that you aren't as lubricated as you should be when he enters...