Well, first of all. Has she lost here virginity yet? Sometimes girls may bleed when they have sex for their first time. This is usually quite common, this should stop after a while, if not. Tell your GF to see her doctor, they've heard it all from everywhere around, im sure they'l find a solution for this.
Does she bleed every time she dances? What sort of dancing? Are you sure that this bleeding while dancing has nothing to do with her period bleeding?
A number of factors affect whether a girl bleeds when she has sex for the first time.
The bleeding usually is the result of the hymen being ruptured by the penis as it forces its way in.
However, hymens vary:
- Some girls are born without a hymen
- Some hymens are very thin, and don't bleed when ruptured
- Some hymens are elastic, and don't rupture on first sex
- Some hymen perforations are large, and a smaller penis does not rupture it
- Some hymens have been ruptured by medical examinations
- Some hymens have been ruptured by vigorous exercise or horse-riding
Hope this helps