SERIOUS GIRL QUESTION

Okay so my period is five days late and for the past week or so everytime me and my boyfriend had sec it was... Well, different. It felt different. Like there was pressure. But now a bad pressure, a good pressure. I don't know why its been like this the past few times we've had sex. And its definitely not an orgasm. I just don't know what would cause me to feel pressure down there when I'm having sex. I mean the sex has been oddly amazing. Better than it has ever felt. I just don't know why it feels different. If someone could please let me know of anything that could be causing this please just let me know.