Well you told that your girlfriend had normal periods on 13ty October. ,and you had intercourse on 17 October. i.e just after her periods were nearly over. Well i dont think she got got pretty chances of pregnacy if her periods are at monhly interval, because this is out of her danger period. the bleding might be an continutipon of her cycles which stared from 13 th october since it is only 5 days.
but i woul ssuggest you take barrier methods
like cions=domsand dont take nunneccdsrty risks. bue take care