So, we can't say for certain what's causing the bleeding, but we can help you narrow down some likely causes. If you noticed pain during sex, then it could very well be that you got a cut or tear during sex. See, what most people think of as the hymen is actually called the vaginal corona, and is a flexible membrane rather than something that gets "broken" the first time someone has vaginal sex. You can read more about that here: My Corona: The Anatomy Formerly Known as the Hymen & the Myths That Surround It
. And while you're not "supposed" to bleed the first time like many people believe, if you're tense or not using enough lube you can get a small tear in the corona or other part of the vaginal canal that can lead to bleeding.
That being said, if you're bleeding so much it looks like you're on your period, then you may want to check in with a healthcare provider. Are you noticing other symptoms, like cramps or burning during urination? And did you use any kind of protection when you had sex?