Sometimes a blood HCG BETA can be off... I had the same thing happen with my first child, and I was almost 5 months when I found out with him.. ( I have irregular periods) ...
make sure to take the test with first morning urine too!
Call your doctor, or test again with hpt. Make sure used the digital one.
Blood tests are not as sensitive as some home pregnancy tests. I would get a first response pregnancy test or a clear blue digital and test again, then call your dr.