What Does A Cluster Of Brown Spots On Penis Indicate?e

I am very scared as I recently discovered a small cluster of brown wart-like spots on the underside of my penis. I have been tested extensively before my last relationship (as did she) and we were both 100% clean. I have not had sex or any form of it since. I often take baths in a tub that could use a cleaning (it's not filthy - but it's hardly sparkling clean). On rough nights, I have passed out in them from drinking. I wake up hours later and only after doing that recently did I notice these things. I am honestly very scared. Any help would be appreciated. I did my "homework" online and the closest I could find people described itching, burning, redness, swelling, or discharge. I have none of those symptoms. Please help.