I just finished masturbating and I noticed some red spots on the head of my penis. I masturbated earlier and as far as I can recall they weren't there. I haven't been sexual active in months.
Looks like balanitis, so these links may help:
https://patient.info/health/balanitis-leaflet
https://patient.info/doctor/balanitis-pro (written for doctors)
Washing (with water, no soap), daily and after taking a leak and washing after masturbation or sex will probably help, and you could also try Canesten cream.
Have you figured out what it is? Please keep posted since I've come across something similar myself.