Genital swelling after sex can be due to friction, or an allergic reaction to the condom (latex allergy). Try switching to a non-latex/polyurethane condom and see if it still happens. If the swelling is painful, getting bigger, or has discharge or fever, it could be a Bartholin gland issue and needs physical examination.
Next Steps
Consult me on Practo or see a gynec if swelling persists or is painful.