For years I slept mostly on my left side. For the past few years, if I do, I get an intense pain by the inside lower breast area. If I push on that area, it is sore, but it doesn't bother me unless I pay on my left side. It's like a severe cramp or something? I have a pretty high threshhold for pain, but when this thing happens, it is horrible. I roll over onto my back and it slowly stops and I go back to sleep. What would cause this?