ER doctors seem to always diagnose "threatened miscarriage." Can't tell you how many times I've heard stories about that exact same thing, and even of ER doctors prescribing mifepristone to help it along, and the doctors were wrong. Unless you miscarry *while* in the ER, I wouldn't ever trust what an ER doctor says until/unless an OBGYN confirms it.
Cramping in early pregnancy is very common. Light bleeding can be, though the clot is more concerning. But unless you start bleeding so heavily that you're soaking through pads, it's still possible that you've been misdiagnosed. Your OBGYN will be the one to trust on this. From now until Monday, just try to rest and stay hydrated with your feet up, and hope for the best.