Doom, I had/have the EXACT same problem. A few weeks into ABBH I my left pec/armpit area freaked out on me. I've heard that the sternum pain can result from going too low. As for the chest injury, I've pretty much decided to lay off of chest stuff until it gets better. The soreness/tightness has progressed from directly under the armpit to slightly more onto the pectoral muscle itself, but that's pretty much the only change.
Did you have slight shooting pains down your arm after it happened? I know that's what freaked me out initially. I didn't have noticable swelling, but I feel you should play it safe and work on horizontal pressing stuff. Heat before workouts, icing any other time. Ibuprofin can help if it's swelling/irritated. Mine's still bothering me ~2 weeks later, but maybe yours isn't as bad.