Hi clopez this could be kyposis wich is an abnormal forward bend in the uper spine, but it sounds as though it developed very quickly so it might be more likly to be a collection of fat at the base of the neck that can be caused by a number of conditions
Causes
Certain drugs used to treat AIDS
Extended use of certain steroid medicines, including prednisone, cortisone, and hydrocortisone
Extreme obesity
Hypercortisolism (caused by Cushing syndrome)
Osteoporosis may cause a curvature of the spine in the neck called kyphoscoliosis.