If you don't have a chancer on your penis there is no risk check your penis daily any concern then see a doctor anxiety can make you feel like you have something you don't have. Have a look at the expert part of this forum indications of what you can have and percentage chances on there if you read. Oral sex is low risks. Stop worrying and go and get tested this will make you feel better. As you will have an date to your anxiety
Receiving oral sex can actually expose you to syphilis. It's also a way to get gonorrhea and herpes. General probabilities are low compared to anal/vaginal penetrations, but not zero. If you concern, you can test as follows:
-3 weeks after the exposure, for gonorrhea.
-6 weeks after the encounter, for syphilis, and if your result is negative, you should repeat the test 3 months after the exposure, as there are people that take longer to show a positive result.
-3 months after the encounter, for herpes.
All the best.