What Causes Red Bumps On The Tip Of The Penis?

did not notice until a few day ago, that i got a few red bumps on the tip of my pennis. they do not have bad odor, nor it hurts. not sure if it its irritation because it happen after last friday when gf and i had sex without condom.i have read similar cases, and the closes is maybe a physical reaction because the intercourse(hard) and her juices produced me a reaction. i highly doubt we both have a STD. does a blood test shows that?