I developed a sore throat a day after giving oral sex. my sore throat went away 2 days later. what could be the cause of this?

Viral: It was probably viral sore throat that resolved spontaneously.

Trauma or infection: Std's can be spread through oral sex, too, so if your sore throat persists, have a provider look and get a culture.

STD?: There are a whole lot of sexually transmissible diseases which can be passed on by oral-genital contact or you just might have caught a cold. It's why you need to consider protection for all kinds of sex, whenever body fluids can be exchanged. If you're concerned, have this conversation with your doctor. S/he can assure you of privacy whatever it is.