Blood pregnancy test wrong?

Shania

I had my blood drawn to test for pregnancy on the 7th because I was having horrible cramping and lower back pain. I also had an ultra sound done and was told I have polycystic ovaries. Never had an issue with my ovaries until now. The blood test came back negative but I was due for my period on the 13th and never started. Today I took this pregnancy test and came back positive. Has this happened to anyone before? I have an appointment tomorrow with my gynecologist to confirm pregnancy.