When we say "skin to skin" contact, we mean oral to genital, genital to genital, or genital to anal contact. It's not arm to stomach, or genital to stomach contact, or anything like that.
The skin on other parts of your body is too thick for germs to penetrate. You can't get an STI from contact with your stomach, arm, back, etc.
You had no risk here. You do not need to test. Whatever "what ifs" pop into your head, nothing with change this. Her genitals can't have come into contact with yours if she was holding your penis in her hand. There is no risk from handjobs.