Yes, it is possible that she tore something inside and that is bleeding. Alternatively, is she really sure that her period was over? Sometimes there is some ebb and flow to it, maybe the rough sex just sped things up a bit. One last thing -- how old is she? When women get older, their periods sometimes get very irregular in terms spacing between them. It can seem almost like one period is coming right after the next sometimes, other times there are long gaps. Ask her if she is really regular, or not. If not, that might be what is going on.
Regardless, if she is worried and it doesn't stop soon (like by tomorrow), then encourage her to go to the doctor. If nothing else, it will help her feel better if she's worried.