It could be the baby pressing against your lungs. My son constantly kicks my ribs which puts a lot of pressure against my lungs making it hard to breathe at times..
Sounds pretty normal for 2nd half of pregnancy but really depends how bad it's making you feel. If it's just discomfort, it's probably not a big problem, but if it's causing lightheadedness or dizziness or changes in blood pressure or body function, then probably get it checked out. When I was really low in iron I was getting really dizzy and breathless and finding it uncomfortable to breathe deeply, but I haven't been getting it since my iron has come back up a little. I still can't breathe that well if bub is pushing up into my ribs, but that's more normal and can't really be fixed.
How far along are you? I've gained 50 lbs during this pregnancy...I'm 38 weeks...believe me...it gets difficult to breath throughout pregnancy.