What Does A Bump Above C Section Scar Indicate?

I had a c section 3 weeks ago, and since then have been healing well according to my doctor. I started noticing this week that I had a small hard bump above my scar that painfully hurt when touched. Well today I noticed that bump has changed color almost like a pimple changes color when it is about to burst. Is this an infection or really nothing to worry about? I ask this because this is my second c section and I had no problems like this with my first.