I have come across this before and if it is Tendonitis then this may be a Repetitive strain injury due as a result of your employment or indeed if it turns out to be the rotator cuff this too can be caused by your employment and both of these can result from repeated physical movements which do damage to the tendons,nerves,muscles and other tissues.The damage caused can be serious and cause great pain and in some cases disability.Many occupations can give rise to this type of injury as a result of the tasks performed each day.
Injections may help alleviate the pain but if it is the rotator cuff then you may be looking at an operation and you have been wise to state yourself this may be a big problem in the future.
You may have a basis for a claim for your injuries whereby you can claim for loss of income,possible loss of future income,medical expenses,pain and suffering,reduced employment prospects,out of pocket expenses and effects on your domestic and social life.
I hope i have been of some help to you but if you want to contact me to discuss further please feel free to do so.