Over The Span Of Three Years, A Strange Lump Appears

Over the span of three years, a strange lump appears on my body. It isn’t any bigger than a dime. And there is never more than one at a time. It is covered in normal skin,not thin, until it is broken open. When it is open it looks as if there is puss but it will not come out. Keep in mind there is nothing blocking it. I can squeeze it, and what ever is inside rises, but returns to its normal place. It always leaves a deep circular scar. So far it has happened twice on my leg, once my finger. Please help!