A little worried

I'm 21 weeks pregnant, and for the past week or so, I've been feeling this weird sort of bubble like feeling in my chest, right around my heart and lungs. It started out only happening when I laid down, now it's constant. I've had acid reflux and heartburn almost all my life, so I know it isn't that. Along with it, I'll get a thumping feeling on the right side of my head, almost as if I can feel my heart beat in my head, as weird as that sounds. I'm REALLY trying to not freak out about it, but it just doesn't feel right. I'm wondering if anybody else has had any similar experiences?