Welcome to the Forum. Your risk from the exposure you describe for any STD, including herpes, is very, very low. The lesions you describe in no way sounds like HSV. After 3 days the lesions would have been expected to have evolved to develop small blisters. I don’t doubt that you have just noticed this spot but, sometimes after an exposure that, in retrospect, one wishes they had not had, persons tend to examine themselves and be far more attuned to genital sensations than in periods when they are not concerned. This in turns leads to noticing what turn out to be normal sensations that might have been not noticed or ignored at other times. This may be contributing to your problem.
As to what it might be, I cannot say. there are many, many normal findings and many non-STD dermatological problems that can cause penile skin lesions. My advice would be to go to your local STD clinic and have them take a look at it. Alternatively, you could ask your own doctor although, to be honest, in most places the people who owrk in the STD Clinic have seen more penile rashes than most other health care providers. At that time you could also get yourself tested for other STDs, again, mostly for reassurance.
Hope this helps. EWH