You need to go see your doctors
">doctor. This sounds a lot like something called a pilonidal cysts. The pilonidal cysts is a collection of pus and other inflammatory material in a cyst between the buttocks. It probably develops when an ingrown hair forms (this is a hair that curls in on itself and grows back into the hair follicle) and subsequently becomes irritated due to the pressure of sitting on it, leading to the formation of a pus-filled cavity under the skin.
These can be quite painful, and as in your case if it is growing and not getting better, then you need to go to see your primary care doctor
. This is because the cyst will most likely need to be lanced and opened up to drain the pus. This is a simple procedure that can usually be performed in your doctor's office after placing a small amount of numbing medicine in the skin at the site. After draining the cyst, your doctor may also decide that you should take some antibiotics to help further with reducing the infection, but this depends on how red and inflamed the surrounding skin is.
Get in to see your doctor as soon as possible!