I find I get them when I am leaning back and relaxing...ie if I am leaning back on the couch. Makes me feel a bit breathless.
You should avoid doing that anyway. I was told it encourages the baby to be postieria. I use to sit like that all through Glenns pg cuz he was pushing so much on my sternium It felt like he would snap it off. He was postieria & Midwife friend told me sitting like that was probably the worse thing I could have done.

Hang on just re read what I quoted. Now just to be clear I would actually sit with my bum on the edge & lean right back with a pillow under my back, so I was kinda arching back KWIM???