If you had said a urine test, I'd suggest that there would be a reason, but (failing a lab error or flat-out mistake in reporting the results to you), a blood test is pretty much impossible to fool.
I would take another test blood pregnancy tests can be wrong I have a 6 week old that we were shocked by because I had a negative blood test, but a $1 pregnancy test from dollar general picked my pregnancy up 3 days later.