Sudden itchy back.

[omits]

Occasionally my back suddenly starts itching. It may be when I go to bed or after a meal. Difficult to pin down. This itching only lasts max half an hour the its gone. It's mainly my upper back a top of arms. Not my whole body like I might have a liver problem say though not sure. Really weird. Had it for quite a long time. I'm in my 70s. Interested in anyone who has this problem and maybe tracked down the cause. Many thanks.