I have recently had oral sex with my girlfriend and we did continuously for about 4 days my vagina was swollen one day and now it burns slightly?

Infection possible . Oral sex introduced bacteria from the mouth onto the genital skin and can increase your risk of infection, especially if there are any abrasions or cuts on the tissue. Friction and moisture can disrupt the normal genital bacteria and lead to an infection as well. It's advised to seek medical therapy for this.