Chemical pregnancy? Birth control shot prevents pregnancy for a year and a half??

I went in to get on birth control. The doctor if course made me take a PT. It came back +. My fiancé and I were so excited! Unfortunately when I went in for an ultrasound they saw nothing and after having blood work done saw my hcg level had dropped. She called it a miscarriage but technically nothing had "planted" yet. This scares me! I got on birth control after that (the shot) I've been off the shot now since October. We have had unprotected sex and even been trying to conceive and nothing is happening. I'm afraid my body just doesn't work??