NO STDs will show symptoms right after finishing the sexual intercourse. It could be just because of the physical action of performing oral sex. That kind of sexual intercourse would expose you to syphilis, gonorrhea and herpes. The real risk is really low, but if you are concern, you can test as follows:
-3 weeks after the encounter, test for gonorrhea.
-6 weeks after the encounter, test for syphilis, and if you get it negative, you can repeat it 3 months after the encounter, as there are people that take longer to show a positive result.
-3 months after the encounter, test for herpes.
Try to relax.
All the best.