What Causes Pimple-like Bumps During Periods?

I have small red bumps very similar to pimples that show up on the last day or so of my period. Some are around the hairline and under my ear. This only occurs on the right side of my head. If i squeeze the bump nothing comes out of it but it turns darker as it starts to go away-very similar to a blemish. It seems as soon as it starts to clear up it starts all over again with the return of my cycle.