What's this lump?
My girlfriend had found a lump inside her vagina. What could this be?
Has your girlfriend ever had a pelvic exam? If no, or if not lately, then she should visit her doctor and get checked out. That's the only way she'll figure out what that lump is. The best I could suggest is it may be her cervix. That's the lower tip of the uterus, and it feels like a rounded bump at the end of the vagina. Otherwise, it could be the G-spot, which is located on the ventral (belly-side) wall of the vagina, about 3 or 4cm in from the vaginal entrance. But there's just too many possibilities for what it is, so her best bet would be to just see a doctor.
In the meantime, you and your girlfriend can read the following articles: