A lot of us get that. WE assume that it's rigidity of the muscles used in breathing, especially the diaphragm. When i first got it one night, i freaked out becaus ei thought the intense shortness of breath was due to CPOD (cardiopulmonary obstructive disorder). Watch this, several years later after noticing this, i spent some time in hospital with pulmonary embolisms (blood clots in the lungs); which i think were brought on by too much physical activity over a short period of time, after spending an inordinate time in bed.