i take the tylenol 3 too for migranes, and i too think its probablky because you fall asleep in a funny position because they make you tired. :)
I also had irregular heart beats that caused chest pains as well as extreme fatigue and shortness of breath. The doc called em heart palpitations and said it is extremely normal to have them because our blood circulation increased so much but with extra symptoms like mine we found it was because I was extremely dehydrated. Now after finally getting rehydrated I have had no reoccurrence of the palpitations and am feeling great again!
I get that pain all the time from the position I'm laying in when sleeping. I also take a Rx pain meds (vicodien) and I have found that because it makes you sleepy when you are in an uncomfortable position when sleeping you may not wake up from it so when you do wake up you have been in that uncomfortable position long enough it's painful. That's what I have noticed. I'm not sure if that's what's happening to you as well.
Maybe just call the dr If it happens again to see what he thinks about it.... I have a lot of irregular heart beating and a lot of chest pain I'm waiting till I see my ob to ask about it.... I asked my family dr about it and they didn't give me any advice....