Bump already?

Elizabeth • 25 years old. TTC #1 after miscarriage in 2012
I'm currently almost 4 weeks and today I noticed my pants weren't fitting normally. They were EXTREMELY tight. I even had to unbutton them in public and run to the mall just to buy a larger pair. Is it possible that I could already be getting a bump and therefore my clothes aren't fitting? It seems a bit early to me.