There are many reasons for spots - bacteria, fungi, viruses etc. Use a basic non perfumed soap and water, or skin wash and water and keep the area clean. Rinse the soap off well, then pat dry with a piece of non fluffy gauze or paper every day (thrown away after use) You could always try something like Savlon on it. If it bothers you that much put a 'sensitive' skin plaster over it, but don't let the spot stay soggy.
Given that you have had this 'thing' for a few weeks now I would take some professional medical advice as there are a hundred questions about swelling under the skin, on top of the skin etc and obviously nasty puss/gunk stuff can get in it
(get a mirror to look or a very good friend to look/get a pic) There is such a thing that can occur which is an infected hair follicle (pilonidal sinus) That is at the base of your spine inside your bum cheeks. This might take a bit more care and medical intervention - so if it is not clearing up or keeps recurring its worth getting some proper medical advice.