Well their are a couple muscles in there. You have the pec major and the pec minor, and then the intercostal or rib muscles. I have strained a couple intercostal muscles, but I did it because I reflexed so hard and quick to my side that I pulled a two or three of them in my lower ribs. I haven't heard of anyone ever straining an intercostal muscle pitching. I can't even imagine how that would happen to tell you the truth.
The most likely possibility, if it is a throwing injury, is that you have a slightly strained pec (pectoralis) minor. The pec minor is a much more important muscle than many people realize. It "pulls" your scapula, shoulder blade, downward and towards the middle of your body. Basically, it holds the shoulder blade where it is. The best way I can imagine you straining this muscle is by having deficient strength in the muscles surrounding your scapula and shoulder.
The best question I could ask you is if your throwing shoulder is if you would consider your shoulders to be straight or rolled. A picture would help. This wouldn't tell me if you strained the muscle, but if you have deficient "stopper" muscles this would be evident.
Do the Thrower's 10 found here.
Try 2 sets of it. Tell me how much your scap burns on a scale of 1-10 with 1 being not so much and 10 being ridiculous burn.
I'm not a doctor and I don't claim to be, so me telling you that you strained your pec minor doesn't mean that's what actually happened. It's an educated guess, if you will.