What diseases and or disorders would cause extreme bone density loss at 28 yrs old?

Many. Hi. Many disorders cause poor bone health, including cortisol overproduction (Cushing's), vitamin D deficiency (osteomalacia), disorders of calcium and/or phosphate metabolism, chronic kidney disease, GI malabsorption, genetic diseases of bone proteins (e.g., osteogenesis imperfecta); medicines such as prednisone, used in many conditions, is bad for bone, as are phenytoin and carbamazepine.