Tightness after period where I cant have sex. Uncomfortable when wiping after urinating.

I'm asking for my girlfriend. This past week she was on her period. It was a normal period with regular flow and lasted for 5 days. She also has been taking birth control combination pill since January. We are each others first sexual partners, and have only been sexually active for 6 months. Anyways, we tried to have sex this weekend, just 2 days after her period had ended, and after 20 minutes of foreplay and ensuring adequate lubrication, she was experiencing mild pain when I was penetrating. My penis is average size and I'm always very gentle and slow when I'm initiating intercourse, but we had to stop because it was too painful for her. She said that the initial stretch for my penis to penetrate hurt, as well as the stroke outwards. We waited until the next day and tried 2 more times, with the same result. However, that same night we tried a 4th time and had no problem having intercourse with absolutely no pain. I'm not sure why the 4th time had a better outcome, but I always make sure to do alot of foreplay to warm her up before trying any penetration. So we were successful, but now she is telling me that whenever she wipes after urinating, it is "not painful, but just enough that its uncomfortable." She said that wearing underwear doesnt give her that same feeling, just when she actually touches her vagina. I hope this was enough information to help, if not please ask for whatever you need. Thank you.