Are you on any b/c? Is it possible that you are having a m/c at all? I mean, could be nothing, just your body doing a nice big cleanse, like when you come off the pill or something... I don't know, if it's unusual for you I would probably see a doc...and if possible, wrap up the big clot in glad wrap or something (you sound like me, more curious than squeamish about this sort of thing) and take that in with you, because they may be able to test it if something is amiss and see what is up.