periods and sex

So I've been very, very irregular all of my life. I have a period like once every 6 months. But last night my bf and I had sex, and I woke up this morning with a lot of blood in my underwear (pretty sure it's period blood). But I just had my period a month ago. Are my hormones leveling out because of the sex? Or did we do something last night that would cause bleeding? It's enough to fill a tampon, so I don't think it's implantation. I just had none of the signs my period was coming and it really took me by surprise. Anyone else experienced this?