It could be due to sleeping posture, try to switch to a comfortable position for yourself and see if it helps. I'm at Wk35+3 now and i realize i have to just lay flat to sleep comfortably instead of sleeping on my sides. I'm experiencing heart palpitations in the middle of the nights, just have to keep calm and it gets better later.
It's normal during third trimester but if it's becoming more frequent then you need to go see a doctor
i personally recall frequent heartburns