Well, the good news is you don't have any other issues like discomfort, pain or itching. It could be inflammation of sebaceous glands induced by friction. Any chance you can ask your doctor about it? Another possibility is a fungal infection.
I'd say it is inflammation of sebaceous glands induced by friction if you say it really seems to come about due to heavy use. If it's not causing you pain or itching, I'd think it's not a big deal. Do you do for an annual doctor's check up? That might be a time to check it out. However, then the spots might not be there so maybe better to try to get seen when they are there. I would not be overly worried but am sure you'd like to know definitively what is going on.