Acute cramp on reaching which involves rotation of arm.

For the past 6 months I have been getting cramp in my left arm ... initially as I was currently under fairly long term treatment for a shoulder issue (subacromial space inflammation) this was assumed related and would disappear once shoulder resolved.
The shoulder issue has reduced considerably .. but cramp is if anything worse.

If I reach behind to touch behind my shoulder or if I put my hand behind my back - with forearm parallel to ground ... it will go into a camp spasm .. as well as the usual pain, the muscle on the forearm bulges up, almost like body builder bicep flexing.
This also sometimes happens when I reach to pick up a cup for example.
There is no loss of strength or grip in my arm.

I am not sure of the correct name for the muscle concerned .. but runs diagonally from inner elbow over towards mid point of posterior of forearm.
Are there any exercises/stretching that can be recommended to improve this

I have been non-technical but attach a picture where I have marked with pen the area concerned.

Attachment 1295