My girlfriend suffers swelling & pain on her labia during 1/2 our intercourse sessions. I am always gentle & she's mostly lubricated. What's wrong?

Pain with sex. The most common cause of pain for women with sex is vaginal dryness. If she is well lubricated, then the possible causes to consider are vaginal infections (yeast, gardnerella, trichomonas), pelvic infections (chlamydia, etc), latex allergy (if you are using condoms), or cysts, which can occur with blockage of some of the mucous secreting glands in the vaginal area.
Depends. She should see her md. There are many things to consider. If you are using condoms a possible allergy, or there may be possible infection.