Health question.. rotator cuff?

Something I never heard of before about 15 years ago. Seems real common now a days.
I have been having trouble with my right shoulder, and in talking to my daughter in law nurse and my wife's cousin (who is a PA) as soon as I described the issues that was the first thing both of them said.
I did go to the chiropractor a bit between turkey day and Christmas, about 8 visits ( she never mentioned any specific issues) and it dulled the intensity, but I think it hurts more often now.than then, though. Pretty steady dull pain, if I move " just" wrong (right?) It's like someone stabbed a voodoo doll of me in the upper arm, pain gets real intense for a few minutes.

Who knows what about this problem? I'm 55, been a mechanic all my life, most I know who have had this issue have been in a accident, fell on it, etc but I didn't "do" anything like that. It just started hurting one day. Laying on a creeper and putting my arm over my head even with nothing in my hand(weight) like when I roll under to do a DOT inspection on a truck really nails it. Or I reach out to grab the cat, when I draw my arms back in towards my body sometimes seta it off.