Seems u are having a sensitive area where the panty elastic could be rubbing each time, or a bikini waxing or shaving hair ...all of these leading to a small pea size Infection possibly on a sebaceous gland or a lipid tissue which manifests as a pus like thing on a hard lump, u need to see that those reasons are avoided ,wear a panty higher or lower to that panty line and avoid any chemicals or razor particularly there. You may use valbet ointment or sofinox or silverex ointment ,take a course of augmentin Antibiotic ,and revert back ,may be in the end that small troublesome lump needs to be excised as well.