I had exactly the same thing in one shoulder a couple of years ago and was told that it's something that is quite common as we get older and tendons and joints wear. The reaching up and back is the key. I was also told NOT to try stopping doing the things that make it hurt because if you do that you end up with 'frozen shoulder', a very common thing, when everything stiffens up due to lack of use. You can use anti inflammatories. I rubbed comfrey ointment on. It gradually went away. Recently I've had it mildly in the other shoulder but that is going away too.