Virgin gf she s not but feels like it?

ok every time me and my gf have sex she literary gets tighter we don't usually have sex for about a week and 2-3 days or so seeing as we are busy with are daughter but still she just keeps getting tighter and tighter and tighter not that i m complaining its awesome but she always ends up in more pain then hell why does this happen and how can i keep it going with out the pain but really i don't want it to stop just u no her pain my gain makes ME feel bad so ya any comment mit help ...............ha