Receiving oral sex exposes you to gonorrhea, syphilis and herpes, but the risk is really low. If you are concerned, you can test as follows:
-Gonorrhea, 3 weeks after the encounter.
-Syphilis, 6 weeks after the encounter, and if you get a negative result, repeat the test 3 months after the encounter, as there are people that take longer to show a positive result.
-Herpes, 3 months after the encounter.
All the best.