Thank you doctor, much appreciated.
Welcome back to the forum. However, you are asking a question that really should have been directed to the doctor who is treating you.
It seems likely you caught nongonococcal urethritis (NGU) or perhaps gonorrhea fromt he oral sex event. Your doctor has correctly treated you for both of these -- cefixime against gonorrhea, docycycline against NGU (using American spellings). The metronidazole probably was not necessary; it is intended to treat trichomonas, but that infection is not likely from oral sex. However, it won't do any harm, and your doctor was probably just being careful.
Your doctor correctly told you how to take the drugs. The large single doses of cefixime and metronidazole are normal. However, you might wait a few hours afterward to take your first dose of doxycyclines; all three drugs together might upset your stomach.
You should find your symptoms clear up gradually over the next few days. If they continue or you have further questions about it, ask your doctor. It appears he is quite knowledgeable about STDs.
Regards-- HHH, MD