Oral sex is an infrequent source of STDs. Herpes is the greatest risk and gonorrhea is a rare possibility, but neither sounds likely from your symptoms. Keflex would prevent some STDs (gonorrhea, maybe syphilis) but not herpes or others.
I don't know what you mean by "irritation" of the penis. If you have a visible rash, redness, or sores, go back to your doctor. Otherwise, s/he is right; I wouldn't worry about it and do not think you need a second opinion.
Best wishes-- HHH, MD