Hello, A Few Days Ago My New Girlfriend And I

Hello, a few days ago my new girlfriend and I had sex as we usually do. Nothing different added to our pattern of sexual practice, but the next day I noticed I had a few small blisters on the very tip of my foreskin, as I am not circumsized. I have been sexually active with my current girlfriend for over a month now, without sexual activity for more than 3months prior. I'm unsure what it is and I am worried what it could be. It's painless, though noticeable as any regular bluster would be. Any suggestions? I am disinclined to expect it is an STD as she is clean and I've been with only two people before her, both clean.