could well be a miscarraige. if not, then you need to see doc anyway, as passing a clot like that is not good. do you recall when your last period was?
are you in pain? you can take paracetomal
i think you should call docs or NHS direct, maybe hold on to the clot you passed
so sorry, how worrying
re being on the pill: it does have a small , small failure rate when you take it, so you can get pregnant when taking it