Is it normal for my girlfriend's cunt to get sore

I am 19 and my girlfriend is 18. If I want to have sex with her more than twice she says her cunt gets sore from my thrusting, and will only give me a blowjob or handjob for the third shot. My cock is not large, only 6". I enjoy the third time, because I can slide in and out for a long time before ejaculating. Should her cunt get sore from normal sex?