thoracic outlet syndrome?...
Thoracic Outlet Syndrome - for those unfamiliar - is when nerve bundles going to the arm...or veins returning from the arm - both through the narrow space between the collar bone (clavical) and 1st rim get pinched....causing problems.
If the nerves are pinched - it can cause pain in the arm or shoulder area, depending on the extent of pinching. (And - this apparently is not uncommon among swimmers...)
Venous blockage is more rare...supposedly only 2 in 50,000 have that problem....and my son was one of the ‘lucky’ 2 of 50,000....he woke up one morning after a good workout in the gym the previous night...and his arm was quite swollen. There was a 12 inch clot in the return vein....and he had an interesting path of recovery. For him - they had to remove about 3/4ths of the 1st rib ...cut from the sternum and most of the way around/behind.