Totally normal pregnancy thing that kicks in big time for many around 28 weeks according to my Obstetrician who had an evil glint in his eye when he told my DH (when I was 14 weeks and suffering breathlessness) to just wait until 28 weeks when I would be snoring like a trooper!

It's related to the more blood volume thing and all the mucous membranes having increased blood flow. Hence you may find you have a stuffy nose in later pregnancy.