Rimming is considered fairly low risk. You might get genital hsv1 from that, if you don't already have hsv1, or syphilis, but the chances of either are low.
Gonorrhea is a theoretical possibility, but gonorrhea resides in the throat, and with rimming, you aren't exposed to her throat. (Think of oral sex - your penis would be in her throat if it was unprotected, exposed to gonorrhea, and chlamydia, for that matter.)
If your irritation doesn't clear up within a few days, see your doctor. It could easily be hemorrhoids or something, and not an STD.