What Causes A Blister On The Penis?

im 17 5'8" 125lbs with no major medical history backrounds. I had unprotected sex with my girlfriend and now i have a small blister on my dick i dont think its an std blister its more like it got rubbed raw i think from her shorts cause they werent pulled off they were just pulled to the side i tried using a condom after i had the sore but it hurt too bad so we stopped but later on we had sex without the condom and im wondering now if something could have happened since its an open sore and having unprotected sex.