LADIES PLEASE HELP

Okay so im on the 4th day of my period and its super light actually barely even there, the second day was very very heavy, i usually tend to bleed super heavy and up to 7 days and i have no idea why now my period only lasted 3 days and was lighter than usual im very worried, i took a pregnancy test on August and it came back negative also i haveny had sex since may the 13th so i cant be pregnant right? But why is my period shorter this month very worried):