Generally big hard pussy bumps on the shoulder, rear, pubic region, legs and underarms are caused by ingrown hairs. Ingrown hairs are not only caused by hair removal, they can be caused by the hair on your head falling out (naturally, in the shower, etc) and planting itself in the skin and growing inwards, thus causing a boil like lump of puss and infection. You will need to see a doctor if this is the case to get the lumps removed as they're under the skin.
*This is just a potential cause, see a doctor and consider other possibilities (especially if you're female as what I described above is most common in men).
*This is just a potential cause, see a doctor and consider other possibilities (especially if you're female as what I described above is most common in men).