Does the bleeding seem to be happening along with cramps, bloating, or other symptoms you recognize as being part of your period? If not, then it's likely that this is happening because of the sex you had. While people are not "supposed" to bleed the first time they have vaginal sex, things like tension or lack of lubrication make it more likely that a small tear will happen during sex that results in bleeding. Do either of those things sound like they apply to you?