What Could Itchy Bumps/lumps On Penis During Erection Suggest?

hello, iv got a bit of a unusual case and hopefully will be able to see a doctor within the next few days. randomly at times while erect, a small red bump will apear on the head of my penis and will last only about 20 min before its gone. its painless and fluidless. my penis will also itch on the underside of my penis but not neccisiarly at the same time the lump appears. i assumed the worst that its herpes but its unusual in that in last only about 20 minutes before its gone. i had unprotected sex a year ago, and ended up with scabies and tested negative for herpes but sicne the test was done about 3 weeks after it all happened my body might not have produced the anitbodies in which the test reads. i was swabed aswell but still negative. any ideas???