I'm thinking a couple things...
first of all, when you have sex near the time of your period, often the uterine contractions from orgasm will push some blood out (even if you thought you were done). This happens to me all the time, as I have no qualms about having sex during my period lol.
secondly, it's really easy to tear or scratch the vaginal walls during sex or foreplay, and if you use tampons a lot of times they dry out the vagina so maybe you aren't as well lubricated as usual?
But since you are also having cramps and stuff, I would go so far as to say that it's not related to the sex you're having lol. I mean, maybe the first time it was, but this second time seems different because it's heavier and is accompanied by cramps. Bleeding from sex is common, but not heavy bleeding and not cramps.
If sex continues to make you bleed, you should see a doctor. It can be indicative of other conditions.
I hope that helped at all.....