A pilonidal cyst is found near the tailbone or top of the buttocks crease. It may look like a pit or small depression. In some cases, it may have a hollow tunnel (sinus tract) that connects it to the surface of the skin. Normally, a pilonidal cyst does not cause symptoms. But if it becomes infected, it can cause pain and swelling. Causes: The cause of these cysts is not clear. They may form when a hair is forced under the skin or when a hair follicle ruptures. Or, a person may be born with the cyst.