This could be a tricky one. Has your girlfriend commented on it herself at all? That she's finding intercourse painful or anything else?

As women age, some of them experience vaginal dryness. The dryness can be related to any number of things. Insufficient arousal, lower estrogen levels, decreased sexual activity.

If your girlfriend is finding it to be a problem, then a trip to her Dr. would probably be in order to determine the cause and examine methods to help the situation.

In the meantime there's nothing wrong with introducing some extra lubricant into your sex sessions. Something like Astroglide would work very well.