OK, so since your STD risks don't sound worth losing your mind over, it's probably a good idea to think about this and sort it out. Are you really frightened because you had brief oral with someone, bleeding a little or not, or is what is running the show emotionally that you feel guilty because you cheated on someone? Often guys who write in that they are freaked over possible STD exposure are really just transferring how guilty and scared they feel about cheating. They're worried about getting caught, they feel like a bad person, and it all mixes into obsessive worry about (often slim) chances of having an STD as a consequence of their behavior.
To think clearly and maybe stop freaking out, see if you can sort your thoughts into two paths, one of which would be finding out about your medical risks, and the other being how to keep your relationship good. Try deciding where you want to be with the person you care about, in the long run. Imagine that happy future life. From that vantage point, list the things you have to do now to put you on the path to having that. Maybe it's being on the straight and narrow regarding sex. Maybe it's an STD test. Maybe it's a sincere commitment to fidelity. Maybe it's a proposal that you and she get more serious. It depends on what you want and where you hope to go with her. The idea of getting an STD (and maybe getting tested, if that will calm your nerves) will recede into its perspective if you use larger life-planning kind of thinking and behave according to it. It will get you out of the bad head-space, too, and keep you from freaking out.
I've been in the same boat a few times. As stated above your risk is low but not zero. The concern is if you're having any unprotected sex with your steady partner its unethical. No one can make that decision for you.