First of all, congrats on having a long lasting and fruitful relationship. Its good to know that you guys are thinking of getting married as well. Second, congrats on lasting longer than your girl friend. That is a good thing that you have.
Coming to the pain, the only reasons I could think of is 1. Anatomical - where the dick is larger or the vaginal opening is smaller/ has some deformity. 2. Infection in the vaginal area.
Since you guys have been active for 3 yrs, I can only think of the 1st one. Your dick may be big for her, but not for 3 yrs. Humans are designed to evolve and women are the best examples at it. I was quite big for my wife as well, and she complained of pain. We used lubricants to help us out and pretty soon ( 3-4 weeks later), we were happily having sex with no pain. So your dick is not the issue.
And this is where it gets tricky. If she is experiencing pain, she might find it difficult to reach an orgasm. There is very slim chance that she is actually having pain and an orgasm at the same time. You say you are having plenty of foreplay and using lubricants. So there is nothing I can think off, that will give you a correct answer.
Her symptoms make me think of PID - Pelvic Inflammatory Disease. When ever she has sex or movement of her vagina (either through inserting dick or even fingers) will cause pain. Talk to her, ask her if she has pain all the time or only during sex. If she masturbates, ask her if she has pain during then. You may try to finger her pussy and she if she complains of pain as well.
As a last resort, consult a doctor. Go as a couple and talk to a doctor and explain the situation. It could be nothing, so give it a shot.