On both of my thighs, I have 2 identical pimples, totalling 4. 2 on each. They're a few centimeters apart, and are on the top of my thigh, slightly lower than my pant waist. What could this be? They've been there for quite some time. It started with one, then the other thigh, then one more on each. They leave dark, purplish marks, and are normal pimple sized.