You may read this WEBSITE, for me it has answered most of the questions that I had before:
My M came around 2 weeks after my HCG was 0. The doctor said I can get pregnant after 2 cycles. But since I have appointment with the government for routine check-up by end of January 2011, so I guess I will try again after that.
Please remember that this is not your fault, and there is no specific reasons. Even those who do IVF have the chance to get ectopic pregnancy.