27 weeks and struggling to breath

I'm 27 weeks 2 days pregnant and the past few days I have noticed I am struggling to breath. When it comes on , which is usually quite suddenly I feel like I have an elephant sat on my chest and feel as though i am struggling to get enough oxygen in my lungs. I suffer from pretty severe and uncontrollable anxiety so as you can imagine when this comes on it makes me panic and has me very very stressed! I just wanted to know if this was normal? My midwife is useless, so when I ring her she never answers and I can leave an answer phone message but she NEVER rings back. I've spoken to her and everything regarding this but it doesn't change the way she is. Just wanted to ease my mind a bit and find out if this is common?