All 3 pregnancy tests said positive. Now 3 weeks later it says negative. Possible?

I have had the signs of being pregnant, sore , tired all the time, forgetful, hungry, nauseus feeling everyday, etc. I tested (3 different tests) positive on they day after my period was due. Now, 2 1/2-3 weeks later, I started bleeding, which lasted 5 days, and is now done. I still "feel" like I'm pregnant, but a test I just bought says negative. What is going on? My doctor is running blood work tests on me, but I don't go back until Monday for the next test. I'm scared and frustrated. Anyone else had this happened? What was the outcome for you? Any support would be great!