Hello,
It can be due to prurigo nodularis which is characterised by pruritic (itchy) nodules which usually appear on the arms or legs. It is very difficult to treat, but current therapies include steroids, vitamins, cryosurgery, thalidomide and UVB light. If staphylococcus or other infection is present, then antibiotics may be needed.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
This is known as Prurigo nodularis, and treatment is with external application of ultra-potent steroids.