3 positive urine tests, negative blood.

Hi Doctor, I'm here to ask for a clarification.... Last month I had many many pregnancy symptoms which included: painful/sore breasts, tingly nipples, prominent blue veins across my breasts, nausea, food aversions, frequent urination and spotting about 4 days before my expected period. Once my period was late I took 3 home pregnancy tests, all different brands one being a digital test and all three were very distinct positives. Therefore I made an appointment with my GP, who then ordered a blood test- which came back as <1 meaning negative.... Then about a week later I had a very unusually period, it was more intense cramping then normal and had a lot more clots then I'm use too. A couple of the clots happened to be fairly large in size and the bleeding lasted longer than normal.. almost doubled a "normal" cycle. My periods normally last 3 days and this bleeding lasted 7 days. So I wanted to know what would cause 3 false positive tests... or is it possible I experienced a chemical p