My 14 year old son has a sudden spot of fat necrosis that has came up on his hip 4 weeks ago. aprox. 3 by 4 and appears red because the skin is transparent and you see the muscle beneath the affected area. No injury that we recall , No injection site. No bite. No pain, Now the sunken in area is getting stretch marks all within the boundries. A doctor did blood work on it and says that the blood work came back clear. What are the reasons something like this would happen?