(( VaginaPagina ))

It's almost certainly a boil. Boils can happen when your immune system is lower, and they usually pop up right on your underwear line. It's because the staph virus can get pushed into your skin and create a nice little infection in there. Normally your body can fight it off, and sometimes it just can't. I've been getting them since I was a teenager (I'm 22 now). You can let it go on it's own, and eventually it will probably burst and drain, you CAN pop it, and I do sometimes, but be prepared to wear a bandaid on it because the clothing rubbing there will irritate it more. You can also get some tea tree oil from some kind of health store nearby (trader joe's has it, super supplements, etc..) and just put a little on your finger, and rub it straight onto the boil. I would also wear a bandaid over it even if you don't pop it, because it'll be more comfortable for you, and your clothing won't irritate it further.
Good luck!