There will be significant soreness is there isn't enough foreplay. Also if the "session" lasts for a long time it can become frictionally irritated. Great foreplay can make the vagina much more moist. I personally would rather have shorter sessions and more of them than long ones and far between.
Hi,
I am 25 as your girlfriend. i've begun sexuall life a year ago, and i do have the same problem as she has. My bf and i are both clean and when we have sex i am sore for 3 - 5 days and CANT have intercourse till i feel fine again. I dont know what causes it, im never dry, but we (as u) can not find a reason why.
Good luck and be patient.
P.S. i bath with baking soda, but however it takes a week to be normal again.(at least for me)