My girlfriend has irritation?

So my girlfriend and I have been having sex for over 6 months now, but about 3 months ago out of the blue, she said it started to hurt. I thought, okay, that just means I did well, but the pain persists almost every time we try. I'd say 90% of the time it hurts her, and I feel bad to the point where we've almost completely stopped. We went from having it 2-3 times a day to once every three weeks or so. I'm not a pig, I'm inquiring because I'm generally concerned with my girlfriend's health!

We used a condom for the first 2 months or so, and it didn't hurt her. Then we started to not use one (please don't badger me right now about unprotected sex), and all was good for a month or so. Then her vaginal area started to get red and irritated every time we have sex. She said it didn't hurt because she was sore, it hurt because it burned/itched during and afterwards. We have both been tested for STD's, and are both negative for any. She went to a gynecologist and sees them regularly, but to no avail. Their best guess is that it's from the actual motion of going in and out, and the friction is causing irritation. So we tried lube, and it helps sometimes but she says it still hurts about half the time.

I'm wondering if anyone has had similar experiences? When we used condoms, we used Trojans, but once we used a Skyn condom. Now I've heard mixed raves and rants about those, but one woman said that she used it, but then months later she began to get irritated. So could it be Skyn?