What Causes The Feeling Of Ribs Caving Inside?

Since I was young I have had a feeling of my ribs caving in on my right side. no back pain no shoulder pain just a sensation of short duration acute and I have to lean back and try to relax to get it to stop. I feel kind of like I have to focus to breathe. This has happened to me maybe 15 times through out my life. I never know when it is going to happen and I am usually sitting up.