Ahhh I totally get it, rasa macam ada batu kat dada kan, memang common at this stage! A few of my mum friends experienced the same thing around 20+ weeks, rahim yang semakin besar tekan paru-paru sikit, plus hormon pregnancy jugak boleh buat breathing jadi heavier. Try sleep with extra pillows to prop yourself up, and slow breathing exercises really helped one of my friends, tapi babe, since you're getting this quite often and susah nafas, do check with your doctor ye, just to make sure everything's okay, it's better safe than sorry lah.