It may be that all the jostling of your uterus during sex caused a little bit of blood that was left in your uterus to flow out more heavily than just spotting. The other possibility is that your vagina and cervix were a little dry since you were very early in your next cycle. Your hormone levels may not have been high enough to lubricate well and the “trauma” of sex could have caused some bleeding. Another possibility is that you have an infection on your cervix from the Human Papilloma Virus (HPV) that is irritated by sex and causing some bleeding. The HPV can lead to cervical cancer in some women and can be prevented by a vaccine available starting at age nine. Bottom line, if bleeding with sex happens more than once, you should get checked out by your healthcare provider to make sure your cervix is okay.
It's hard for me to pinpoint what's causing the bleeding after sex because there are several possibilities. It could be as simple as a yeast infection or a little tearing/friction that occurred during sex. It could also be a medical condition such as endometriosis, or it could even be a sign of a possible sexually transmitted disease. The only person who can say for sure is your healthcare professional. I don't want you to worry, but I do want you to see your healthcare provider just to have things checked out.
I can never trust the last day or two of my period. I hate it when I think my period’s over and I go without a tampon, only to find out later that my period wasn’t actually over – another pair of panties bites the dust. To me, it sounds like your period wasn’t actually over. Though there may be very little coming out, sometimes, there’s just a little more in there. Even when my first thought is that I’m embarking on the last day of my period, I always give myself a buffer and stay on the safe side by going on extra day with either a tampon or pad. And believe me, there have been times where that extra level of paranoia has paid off.