extra symbol on powerup sign?

Last night was going downhill and had the aero powerup, so I activated it to see if it would make me go faster. I’m not convinced it actually did anything for my speed, but towards the end of the “duration meter” winding out, a pink “PLUS” sign popped up in the lower right corner of the powerup indicator. What does that mean?

Did you pass under the start of lap line at that point? If you have a powerup active when you go under one of the lines, the new powerup appears in a little circle in the lower corner. It expands to full size when the current power up runs out.