Why do i have pimples in my butt?
To remind you to get off your butt and get to work.
Oily skin may cause pimples. Extremely hot showers bring more oil to the skin surface. The other reason may be sitting for longer hours. I would suggest you try using a soap or body wash that exfoliates the skin and see if it helps.
Go to a dermatologist.
Are you sure those are pimples and not ass warts?