Shoulder flexion: Infraspinatus or deltoid?

As mentioned in several previous posts, I have chronic shoulder pain. It's usually my left shoulder that hurts, but sometimes it is the right shoulder. And it hurts in different places depending on the week. Of course it isn't continuous nor severe (otherwise I would go to a doctor), but I still try to figure out what I'm doing to injure myself. My latest bout of shoulder pain is on the left side in a muscle that covers my scapula. It is called the infraspinatus. This muscle uses the scapula as a base and pulls on the upper head of the humerus bone, rotating it away from the body (external rotation). It will also pull the arm behind the body. This muscle hurts right now, and I had a distinct weakness today while doing backstroke. In fact, I really couldn't do backstroke at all.

Infraspinatus is in red, hidden below the deltoid of the shoulder. (from Wikipedia) 

I was about to blame my freestyle technique on causing this pain, but now I'm wondering about that. You see, in freestyle I have been trying to extend my arms as far in front of me as I can to initiate the catch and hold them there for as long as I can between alternate arm pulls (described in a previous post).  This is not a problem for my right arm, as my head is down in the water and facing the bottom of the pool (or actually, a little left of the bottom if I'm doing the body rotation right). But when it is time to hold the left arm out in front while the right arm rotates up and out of the water, I usually take a breath, and the motions of my right arm and my neck tend to lower the left arm before the proper time. As a consequence, the lowering left arm produces unwanted drag and limits the amount of water I can catch and pull with that arm. My coach pointed this out, so I've been working really hard to keep that left arm in place even as I breath on the right side. As best I can tell, the infraspinatus is not involved in keeping the left arm out in front (if I were standing upright, my left arm would be held straight up--flexion). Instead, the lateral deltoid does this in conjunction with the trapezius and triceps. Certainly those muscles have hurt before, but not today.

So I still don't have a good explanation for how I hurt my infraspinatus. Sure, I did some backstroke yesterday, but not enough to do me harm. At least, I think.