I have a small rash the comes on goes on my penis it's there for 2 days and all of sudden disappears then comes back later like 5-6 weeks and disappears last for less then 2 days and then gone again is this something I should be worried about already got tested for stds and I don't have anything, and my doctor says I'm fine, any reason why this keeps happening.?

Contact dermititis from laundry detergent.

Sweat. Use an anti-fungal powder when present.

I'll bet it's the latter.