False pregnancy test results are often caused by not following the instructions exactly (especially on when to read the result), misreading the results, and expired tests. The blood test is the definitive test.
It is very unlikely that hCG will just spontaneously occur in levels large enough to measure with a pregnancy test, without you being pregnant. You can get enough in your blood to test positive from steroid use, ovulation inducing medicines and some cancers. The fact that they did not find elevated levels in the blood test is however an indication that there is something wrong with the urine pregnancy test, not your body.
Keep on working with your doctor to make sure something was not missed.
Take care!