Is she lubricated enough when you have sex? that could be a reason. If she isn't then you can use lubrication or try extending your foreplay. I mean like doing thing s to make her feel relaxed and happy like a long walk and reading poetry, (find out what she likes first)or massages with oils, stuff like that.
Somethings medical science doesn't have a proper answer for yet so I think you should try that as well as checking with the doctor.
Good luck and keep us posted?