I Always Have Normal Periods But For The Last Two Months My Period Has Only Been Lasting Two Day Which Are Lighter Then Usual And End In Spotting What Could Be The Cause?

Jam not sure what is wrong, but this period that just past lasted two day and ended in spotting my normal period is heavy and last up to 5 to 6 days i have never really had and abnormal period. This month that just past i had a few clots in it but im anemic so this is normal for me. And i took a pregnancy test it said negative..I'm confused if you have any info that can help me let me know.