I hope I can help you with this, and I can understand your concern. Unknown lumps and bumps can be disconcerting, especially when they appear on the genitals. Please understand though that online no one can state anything with any certainty, all any of us can do is offer a best educated guess based on what you wrote.
From what you wrote, there are a few possibles that come to mind...for the final answer, however, an in person exam by your health care provider is the only way to truly know the definitive diagnosis. But here are some guesses.
I am happy you had a negative STD screen at two weeks after an encounter. The two week time frame is long enough to detect the STDs I would worry about that could cause a rash that might be somewhat like you describe. STDs that cause rashes include herpes and syphilis; HPV will leave wart like lumps of various types.
These may be FORDYCE spots. These are normal, a variant of sebaceous gland beneath the skin and you would have had them. I know you said you've been carefully examining yourself, but its possible you did not note them prior to this. Information on these and pictures are here: http://dermnetnz.org/acne/fordyce.html
Alternately, these lumps may be Pearly Penile Papules, which are another normal variant in men. Images of them are here: http://dermatlas.med.jhmi.edu/derm/IndexDisplay.cfm?ImageID=-484272306
The way you described these with a white tip makes me wonder if they might be molluscum contagiosum. This is a virus that causes a wart like structure which has an indentation in it which you may be seeing as a white tip. It is a contagious virus, passed by contact. It is classed as an STD but its easily gotten by non sexual means. Children get it often. More information on that is here: http://www.cdc.gov/ncidod/dvrd/molluscum/faq/everyone.htm
And a link to a page with signs and symptoms of common STDs. You will see Molluscum on there; in my opinion it should not be. http://www.fwhc.org/health/std.htm
The only way you will get a definite answer as to what the bumps on your penis are (diagnosis and treatment) is by seeing your doctor. But I hope these ideas on what it could or might be eases your anxiety somewhat.
I hope I was able to help,
Just Answer has recently implemented a new rating and feedback system. Please be aware that you are rating my courtesy and service as a professional, not the site itself.
Please remember to only rate my answer when you are 100% satisfied. IF you feel the need to click either of the two stars on the left, please STOP and reply to me via the "CONTINUE CONVERSATION" button. Let me know the issue you have and I will be happy to continue further and do everything I can to provide you with the service you seek.
If you give any rating below 3 stars then I will not be compensated in any way for helping you today.