Oh, many things. You can easily get small tears or scratches in your vagina or labia minora that will bleed after. Especially, it seems, when the guy fingers you because of his fingernails. If you are not feeling any discomfort, I wouldn't worry about it. However, if you are sore or feel sandpapery than you need to use more lubricant when you have sex.

Also if you are near your period, uterine contractions during orgasm sometimes push out blood.