Since this is getting larger and start to hurt, you definitely need to get in to see your primary care doctor
about this as soon as possible.
There is a form of cyst that frequently occurs in the region of the body, between the buttocks at the top, called a pilonidal cyst. Most of the time, pilonidal cysts are caused by chronic stress on the skin at the base of the spine - for example, from prolonged sitting or ingrown hairs, leading to the formation of a blind pouch under the skin. There may also be some cases of people being borne with pilonidal cysts, but this is less common.
The problem with a pilonidal cyst is that it may occasionally become clogged and infected, leading to enlargement and pain. This is what I am concerned is going on in your case. Most cases of infected pilonidal cysts need to be opened and drained, which is why you need to go see your doctor to see if this is necessary.
Your doctor can determine quickly if this is an infected pilonidal cyst and, if so, if it needs to be drained. They can also decide whether or not you need to take antibiotics to clear the problem up.