I Have Noticed A Hard Small Egg-sized Lump In My

I have noticed a hard small egg-sized lump in my buttock toward the area where the two cheeks come ... a sharp pain sometimes when I sit. What is this?Additional information: Lump is not visible on outside .. And I started a job where I sit all day and I had another one a couple of months ago it finally went away then this one started a couple of days ago