Hi Orangepeel,
We can't say for certain what's causing the bleeding, although if it's paired with cramping and other symptoms that you associate with your period, then it's likely a sign your period is coming soon.
What we can say is that it isn't a sign your hymen is broken because the hymen (or vaginal corona) isn't something that can break. You can read more about why that is in this article:
My Corona: The Anatomy Formerly Known as the Hymen & the Myths That Surround It . It is possible for the vaginal corona to get scratched or tear during sex if a person is tense or their isn't enough lube present, but if that was the case you would have seen the blood soon after you had sex. Does that make sense?