Should I be concerned or not?

My two year old son had a pimple like bump on his back and it went away about a month later it came back in the same spot and had a red looming ring around it eventually the little ring went away but the bump stayed and turned brown. It looks kind of like a mole on his back but I don't think that's what it is. I'm not sure whether to call his pediatrician or not because it doesn't hurt him or anything I just have no clue what it could be!!