This is such a perplexing thing. I post on a clomid site elsewhere, and another lady had the same thing happen to her. She was NOT pg. Unless your blood test was switched with someone elses, a blood test is most accurate.
I was always taught that there really isn't anything that can make a HPT read positive except hcg. You only get hcg in your urine from a pg, an Hcg trigger shot, or a tumor. However, you are not the first woman to have a false pos. I would keep getting beta hcg blood test until your period shows up.
They are not evaporation lines are they? Did they show up in the time frame that the instructions state?