positive clear blue, negative at doctors

I tested using the clear blue digital and got a positive result on Wednesday. I started bleeding yesterday - started off with light spotting but became heavier red blood overnight. Went to have a check up at the hospital and they gave me a scan (I am around 5 weeks) and couldn't see anything other than a "bulky" uterus (this could indicate pregnancy or my period). They gave me a urine pregnancy test and told me that the results were negative. How can this be? The bleed is nothing like my period and is 6 days late. Despite that I got a positive results when I was 3 days late :( any advice would be greatly appreciated xx