Every night for oh, the past 2 or 3 weeks or so I've had difficulty falling asleep because my heart feels like it's going to pound it's way out of my chest! It's maddening.

I KNOW it's not anxiety, and I know it's likely not because I'm overmedicated (i'm on 30mg of Armour for my underactive thyroid, and have been for, 6 weeks or so)
Last night was particularly bad. I REALLY just wanted to go to sleep. I was so tired - it was midnight and my eyes were closing, but my heart was pounding so hard I couldn't fall asleep. I was actually kind of scared..
My pulse wasn't any faster than usual, but my entire body was throbbing with my heartbeat. There was no pain involved at all either.. otherwise I probably would have headed to the ER..
*sigh* Please someone reassure me here - It's pretty scary when your heart beats that hard. I'm 24 years old and It just seems very bizarre to me to even think I might have a heart problem.
The strange thing is that when I would sit up in bed last night, my heart would actually stop pounding as hard..

That really seems backwards to me.
My next doctors appointment is June 13th, and I do plan to mention this to her when we go over my next round of thyroid labs..
Thanks everybody - this is kinda scary