I had the same problem, and I could have swore I had a heart problem. But really I have a Panic Disorder. Panic Disorder can come out of nowhere and be caused by nothing. It causes chest pain, now I'm not saying this is what you have but its worth looking into. Now that I have found out what I have and I'm now on the medication called Paxil its turned my life around. I feel 100% better, occasionally I'll get just a few seconds of chest pain but since I know know what its from Im not scared anymore.
It could be a gall bladder problem, you need to only eat low fat foods, no evening eating, lemon juice helps, also eat beets ..google for the best diet , its food that triggers it. no alcohol, does it start under the ribs on the right and radiate into the back, they will do tests for it and tell you what you need to do .Good Luck
also causes the feeling of unfilled breaths but only on occasion. after at least 8 er visits im desperate for help!!!!!