What Causes Yearly Reappearance Of A Pus Filled Boil On Upper Abdominal Muscle?

I`ve noticed for the past 6 yrs a small, (1cm by 1cm) hard lump like a boil on the center of upper abdominal muscles , which re apper every year with formation of pus but subsidise eventually leaving again a mass.. Any ideas what it could be. i feels kind of superficial for that...like its coming off the muscle.