Why is my period getting shorter?

I’m 21 years old and I am having a problem with my period lately. For the past two months, my periods have been getting shorter. I am not on any birth control at all. I have also lost about 20 pounds within this time period and have been exercising a little more.

With that said, my periods shortened from 7 days to 5 days. I know this is something I shouldn’t be complaining about, but I just think it is strange.