About a week ago, I noticed a small red bump above my right buttocks. At first I thought it was a pimple but there was harsh pain whenever it was touched. Then a couple of days ago, I got two more bites on the backs of my legs. However, these are much more painful and red. They are both very red around a black/brown dot in the center. Extremely painful to the touch, or when I sit down, I assumed they are from a spider but I m not sure.