Pregnant?

I'm 3 days late for my period there could be a very small possibility that I'm pregnant so I took a pregnancy test (clear blue digital) it said I wasn't pregnant. I took the test 3 days ago because of a few pregnancy like symptoms and I was getting a little worried. 
Is there any reason that this test might be wrong as one of my friends took one and it came up negative when she was in fact like 8 weeks pregnant... Help! I'm only 18 and a diabetic and this could be dangerous and I'm not very well controlled.