Sore throat is not a common STD symptom, but could be gonorrhea--the most common STD acquired by performing fellatio. Herpes is theoretically possible, but usually is associated not only with sore throat, but with obvious open ulcers in the mouth and throat.
However, for 30 years it has been known that people who perform oral sex on men have an increased frequency of sore throats in the next few days. Typically no STD is found and the cause is unknown. Speculation has been on minor trauma, physical irritation (e.g., from semen?), and anxiety.
You first describe the sore throat as mile, then "quite painful". You should see a provider, and also ask your partner to get examined for STDs. While you're at it, you probably should address HIV risks. The chance of HIV is low from performing fellatio, but not zero. You should have asked and shared HIV status with your partner before the exposure; perhaps you did that.
Good luck--- HHH, MD
95% of throat gonorrhea cases are entirely asymptomatic.