When you think about it, sex is pretty invasive, I mean, the skin around your
vagina is literally stretched by your boyriend's penis and the tearing of the skin is what caused bleeding.
It is not period blood at all. Also if the encounter was a little bit rough and unlubricated, it is just trauma.
I mean if this is the first time, you have to get used to it.
The size of the penis matters too. If your boyfriend has a larger penis, it is more likely to cause trauma to your vagina.