This has happened to me. Unfortunately, I've had eight miscarriages (four before my son, and four after), and I had one false positive due to hormones leftover from a pregnancy with twins I lost. My ob did an ultrasound to discover that there was no baby, but still a sac, so my body thought it was still pregnant. After that, the ob had me go in for blood tests literally once a week, until the hormone count went down all the way, then we were given the green light to try again. That's when I got pregnant with my son.
In reality though, you may very well be pregnant. I know you want answers immediately. We all would. But just let your doctor do his/her job and find out the right way. No matter how many people respond to this message, your experience is completely different from anyone else's, and the only answer you'll get with any clarity is that from your doctor.
Wish you the best.