Sounds like it could be herpes. If so, you might have had it for years, not from your most recent parnter. See a doctor or clinic right away, the sooner the better for accurate diagnosis.
It could be due to poor hygiene. That means touching your penis without washing your hands or it could be a bite. If your girlfriend has been unfaithful there would be a possible STD but 2 blisters does not seem like syphilis or herpes so I would be surprised for it to be an STD. As long as your girl is faithful you have nothing to worry about, if you are not sure just get tested.
Nonsense. Hygiene problems like "touching your penis without washing" doesn't cause blisters or any other kind of genital sore. And "2 blisters" would be typical for herpes, although not for syphilis.
Thanx for the comments. I'm confident it's insect bites. As I am outdoors on a daily basis in many different environments. And I thought herpes was painful??