My wife occasionally has a sore pimple around her vagina after having sex. i do not have the same symptoms. what could it be?

Female Genital Sores: Female genital sores may appear as small red or flesh colored bumps &blisters.They are caused by STD,and can be caused by another condition as dermatitis.Some sores may be itchy,painful,tender or produce a discharge,and some have no symptoms.Some appear without any reason and resolve by itself.It may be a good idea to get it checked by her doctor or gynecologist to find what is causing it