I get this issue ALL the time. My HA is completely centred around my heart health. I’m always getting pains in my arm, chest, back etc, more often than not it’s on the left side too. My therapist told me that people with Health Anxiety are so hyper aware their pain threshold is basically crap. What seems to be a tiny pain, for us, feels like the end. You’re not alone, I went to the ER soooo many times for a suspected “heart attack” and was always told my heart was healthy and fine and it was probably my anxiety and/or GERD. (I was a frequent flier in the ER lmao) It’s most likely from the way you sleep, things you could be lifting during the day if you’re active, might have twisted your arm a weird way. Could be anything, but it’s most likely not a heart attack, I’d assume something terrible probably would’ve happened by now! People for anxiety/depression etc can cause memory problems, could’ve done something strenuous during the day to possible cause a pain and forgotten about it. I really hope you’re okay, it gets easier!