Why am I getting worse at running?

I started running 2 years ago, nothing serious. just 1 to 2 miles a day. Then a year ago I decided I wanted to do some 5k races. I did 2 and did them in 28 min and 27 min which is not bad considering they were hilly and I am not an experienced runner. Now I just did a 5k and did it in 30 min and could barely catch my breath. Is this normal to sometimes hit plateaus and have bad runs? Should I get discouraged and quit?