When you m/c early you can treat it as a period - I o'ed 2 weeks after I m/c'd at 6 weeks and I became preg from that cycle - and am now nearly 33 weeks. I had had blood tests thou that said that my HCG level were back to zero.
Im not sure if that helps or not. I guess it takes different amounts of time for the hcg levels to go back to zero, sorry Im not much help! Im sure that some one on BB will be able to help you out tho.
Good luck
Kate