Weird Panicky Feeling

Not sure how best to describe this... Over the last week, I get this random feelings of surprise/ panic that rise through my chest. It's like that feeling you get when someone surprises you and you're not expecting it. The weird thing is that it happens when I'm relaxed usually. For example it just happened right now as I was closing my eyes to go to sleep. I was super relaxed, tired not anxious at all.

Has anyone else experienced this during their third trimester? What is it?

I should add that my heartburn is also really bad lately. Not sure if that has an impact at all.