There's a hard lumps under skin on my rear. After lump recedes, it leave a bothersome dark spot. Any suggestion? (photo)

Occasionally, these hard lumps will appear under the skin on different areas of my rear or between my thighs. They do not hurt or anything but then will eventually go away. But they are leaving these terrible dark spots everywhere after they go away. What are they and what can I do to treat/stop them from appearing?

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