if it grows rapidly in size and is extremely hard and painful it could be mrsa or staph infection. however if it is staying the same size it may just be an ingrown hair. ingrown hairs can get infected and become staph so make sure you clean it and not mess with it.
i have a red swollen pimple like area on my stomach that is very sore and is hard (ish) about 3 inches in diameter. it started almost like a pimple (i thought it was) it's been 3 days and it hasn't changed. it may be getting aggrevated and staying red and hurting due to the fact that when i set up my pants belt underwear etc rub against it.