They can be cysts or ulcers (pustules or non-pus vesicles) caused by a clogged or inflamed follicle or sebaceous gland. Sometimes that happens because of too much friction. Bacteria can get in there and make it turn nasty. Using a chamois cream, especially one with antibacterial and anti-fungal properties, will help prevent them. They can occur in different locations for different people. I think a lot of it has to do with where you get the most friction and if you have anything else going on (like ingrown hairs from grooming the bikini line).