I found out something about flu shots just recently.

I was told that it's very possible that the flu shot I had in the fall of 2013, might have been what triggered the extremely painful adhesive capsulitis (Frozen Shoulder) that I had last winter in my left arm.

Had a flu shot about a month and a half ago in my right shoulder, and over the past week or two, my right shoulder has been hurting. It's eerily similar to how things went last year.

Still believe in flu shots, but I think any future flu shots I get will be done in the side of my butt from now on. Flu shots don't have to be done in the arm.

From what I was told, and have been reading. About 10% of the people who get a flu shot, can develop frozen shoulder. They get frozen shoulder if the person giving the shot accidentally injects it into the bursa, or another part of the shoulder. Like they're giving the shots too high up on the shoulder.