Why does my heart pound so hard when I lie down?

My heart thumps so hard at night when I lie down in bed that it’s like I’ve been exercising. I’ve just been reading and then put out the light.

This has been going on for a few months.

I don’t even feel that comfortable on my left side.

Is this any connection to having trouble going to sleep? Again for about the past few months.

Thanks for any advice.

P.S. By then I haven’t had any caffeine for at least 12 hrs.