How far along are you supposed to be? Did they tell you at what point they thing the baby passed? Perhaps it is not really a m/c....
i think i asked before and oyu tod me 'no' that you hadn't had bloodwork done. i would demand this of them! if they didn't run all the test(s) possible to determine thier diagnosis, i wouldn't be satisfied. perhaps you were very early in pregnancy so they couldn't see anything (how far were you supposed to be), perhaps you are experiencing a m/c. have you passed any clot like materials? if you stomanch is hard and you are m/c, it may be your uterus contracting to get back to normal. at this point it is so hard to tell when you have so little answers/help from your doctors and tests.
See a different dr who can do the b/w and run some tests! Second opinion will not hurt.