What might cause such quick muscle development?

Earlier this year I noticed I had no muscles in my arms. I did pushups, did exercises using very small weights, and went on a swing that required a lot of grip every day. In 3 weeks, my arm muscles were rock hard when flexing, and bulgy. I'm a girl, age 15. Is this normal? Like, that fast to gain muscle? I went from NONE to impressive. All my life I've had a pretty muscular build.