Bumps on my thighs that look like pimples, once gone they leave dark spots.

See your doctor.: It's hard to know the definite diagnosis without the lesions being seen. It sounds like they could be secondary to a bacterial infection or an irritation from something. The dark spots sound like they are the scars left from the inflammation. These should fade eventually, but could take quite a while. So, if you are still concerned, please see your doctor or a dermatologist.