This doesnt sound like an STI to me. Could be cause by friction (teeth maybe or since then you are checking it alot?). Potentially balanitis which is not an STI.
I would stop masturbating for a few days and stop checking it. See how it goes. I think it will all blow over and you are good to move on. Dont let anxiety take control.
From YOU giving oral to her is very very unlikely to transmit an STI to yourself due to how STIs work with the female anatomy.
From getting oral from her, it is a bit risker than giving it, but still statistically lower odds and is lieu of symptoms not worth testing over a single encounter and this does not really sound like a symptoms of an STI.
Your emotions about your ex (now current so congrats) are making you feel guilty. You did nothing wrong, you were a single lad enjoying yourself. move on from this :)