The risk here is very, very, very low, if any. The only risk comes from you having the penis in your mouth, and since it was so brief, I wouldn't worry about it at all.
Giving oral sex to someone with a penis puts you at risk for oral gonorrhea and syphilis. Both are fairly uncommon. It would be very unlikely for you to get oral gonorrhea, which usually resides in the throat, if the penis didn't get near your throat or your partner didn't ejaculate.
Syphilis is only transmitted by direct contact with a sore. If your partner had a syphilis sore, and you put your mouth on it, you could get it that way.
Since it was such brief contact though, and syphilis is far less common than things like chlamydia, HPV and herpes, I wouldn't worry about it.
You should know that taking the amox will affect you testing for a few weeks, should you decide to test. Amox won't cure gonorrhea or syphilis, but could give you a false negative on the tests. If you want to test - and you really don't need to - you'll need to wait about 3 weeks to test. If you test for gonorrhea - and again, you don't really need to - ask for an oral swab. Since you weren't exposed genitally, a urine test or urethral swab won't be effective in diagnosing you.
But really, I wouldn't bother with testing if I were you.