For a week now, I have been spotting brown blood, with some spots of red after having sex with my boyfriend. It started right after we finished. I've been reading up on my symptoms and the most results I got was about implantation bleeding. Even though my boyfriend and I have sex regularly (about two or three times a week) we are always safe, so that should not be an issue. I've always had very irregular periods (They're usually about 3 to four months apart from one another.) and have bled some times after sex, though not for this long at all. My last period was last month at the beginning of the month, then we had sex the beginning of this month. Should I be concerned? What could it be?