What Causes Light And Stringy Periods?

I was 3 wks late for my period last week when i noticed blood in the toilet and

then i bled for 5 days. It was not a typical bleeding. I had no flow but the

blood was there any time i wiped and when it dropped out was was light red and

goopy/stringy. i took an at home pregnancy test and a blood test and urine test

ayt the doc and they all show low hcg meaning im neg for pregnancy. what does

this mean?