Lately I Have Been Having Trouble Taking A Deep Breath.

Lately I have been having trouble Taking a deep breath. It feels like I have a cramp in my chest. Sometimes it is right above my heart other times it's right where my sternum is. It feels like I have a cramp in my chest. It feels like an air bubble. Have had it for a couple months now. What could this be? I'm 30 and healthy. I try to stretch to dislodge it but nothing seems to help.