What Causes Black Spots With Itching On Body?

Hi
I have black spots on my butt. I don't know how it got there but it itches very bad. It started off as my butt check was itching in the same spot and a bumps appeared on it. These spots that's on my butt feel like they're swollen up when I touch them. I use everything known to man from over the counter to grandma remedies nothing seems to help. Now these same spots are now working there way up my back. I thought or maybe it could be fungus but not sure. Any suggestion from you guys!!