ive had the same thing since i was young, and was told that it is psoriasis. it is a skin condition to do with the skin cells dying at an abnormally fast rate and there are no cures, however it is able to be kept under control to a point. Getting sunlight on it will help to stop the itching that will come later. you can get a liquid called pinetarsol which you put in the bath and it reduces inflamation. the dr can prescribe various creams, but you need to rotate the creams every few months or they stop workingBe lucky you only have it on your abdomen, i have it on my face, ears, scalp, arms, legs, back, chest, pelvis. Also try to avoid tight clothes as they tend to irritate it, and try to use a sensitive skin soap as it tends to help stop your skin being easily irritated. Stress and illness can mak it worse, and it usually comes and goes but it will always be there somewhere. It isnt actually that uncommon, but they dont know what causes it yet. I would suggest going to a dr or dermatologist as they can help your specific case better as there are many forms of psoriasis
feel free to pm me