I did some googling, and I guess it's caused by progesterone, which makes the diaphragm and other accesory muscles of breathing relax. Not a whole lot we can do about it, apparently. They said it usually eases by the 20th week or so. About the time shortness of breath due to baby taking up lung space kicks in. Ah well, eventually the little bean is coming out of there, and we'll be back to normal again.