90 Degree Squeeze

It seems easy enough so why does this hurt so bad after only a few?  It doesn't take much weight to start feeling this one...

Grab a couple small weights or water bottles for each hand.  Bring your arms up so they are bent at the elbows (90 degrees) and out to the side, like you are going to do an Overhead Press.  Then, squeeze the elbows together in front of you, keeping your arms at shoulder height and the 90 degrees.  Squeeze elbows together as best as you can, then go back out and squeeze your shoulder blades together in your upper back.  Repeat...