You mentioned your doctor doesn't feel you have asthma, so stress inducing your breathing problems seems reasonable to me. My sister had breathing issues for 6 weeks. She had her heart tested, lungs tested... and in the end it was stress causing it. Once she got a grip on her anxiety she felt better. Deal with any anxiety/stress you might have right now and see if things settle down. Reading self help books, deep breathing and keeping yourself occupied doing things you enjoy could all be a step in the right direction.
Hope you feel well soon.