Hi sparkly, I think I get what you're talking about too and it's driving me mad. It feels like a deep tightness that is uncomfortable but if I breathe in really deeply it gets worse. I have tried lots of things, buying bigger bras etc to figure out what is causing it but I'm no closer yet. It tends to me for me when I am more hungry, and especially if I am hungry and drinking cold water. Usually once I eat it goes away for a while.
Sorry I can't provide much advice but just wanted to let you know you might not be the only one! x