Beautiful photo!
I am not a pro or expert by any means, but...
I think it is chromatic aberration.
We were talking about this in my photography class. It doesn't matter if is a cheap lens or not (of course more expensive lenses perform better, but even a very good lens can produce Chromatic aberration under some light conditions ) this is a light phenomenon caused by the inability of the lens to focus all the colors in the same way. And isn't always purple, it can be green, red or blue too.
It gets worse when your subject is lit from behind, so try to avoid that. The best solution to correct it is to shot in RAW. If your file is not RAW, you can correct it in PS using "adjust hue/saturation' or in LR using the "chromatic aberration" option.