Middle back or shoulder blade level pain is extremely common in the computer age and often has little to do with arthritis. Without knowing your occupation, the number of hours spent at a computer, your seating position and a host of other factors, there is no way of knowing if this is your issue but I would prefer you follow up the more likely and common scenarios than start looking for a ‘medical disorder’. There is a real likelihood that you may have some wear and tear consistent with your age which someone may label arthritis and it may have nothing to do with your symptoms.
It is worth noting that we see a large number of middle aged people whose posture is being weakened by the number of mobile digital devices being used without adequate advice or exercise to balance it. This is the most common reason for strained muscles and pinched nerves in that spinal region and it is usually compounded by other lifestyle factors.
Seeing a professional who is trained to assess this condition is what is required. The history and examination will narrow the field and it is very likely to be treatable. I would ask friends or family if they know of a local chiropractor, osteopath, myotherapist or physiotherapist who can assess, advise and treat the issue and recommend an exercise programme for future prevention as well.