I don't know much about bleeding while pregnant and such. But when you're usually having your period have you ever like sat for a long time, or gotten up from bed and it feels like you've peed your pants or something. Just that "whoosh" of liquid? Maybe it was "heavy" because it had been sitting, and even if you weren't sitting, maybe just more "tight" than you are in the shower, then it could've just been flowing right after collecting. Not that you were meaning to hold it in, but maybe you were. Just to be sure though, you might want to call like a nurse-hotline or something. They don't charge and they can give you a good, educated, answer about whether you should worry or not.