"Yikes, I think that I am having a heart attack" or "Wow, babies are heavy!"

So two nights ago I was having this really annoying pain in my chest that seemed to radiate through my left arm to my shoulder and then to my neck. I have had chest pains off & on for two months but I really was trying not to freak out about it, because when I mention chest pains & left arm what instantly comes to mind? Heart problems-right? So for two months I have ignored it. Well Tuesday I was in a lot of pain and it just seemed to get worse and I thought, "Oh geez I am having a heart attack!" I didn't know if I should go to the ER or what to do because what if was nothing. I called my aunt who works for my doctor and she said to just come into the office first thing in the morning.
So I go in and see the NP. He starts to move my arm around and finds the spot in my chest that is killing me (pain wise). Turns out that I had pulled a muscle a couple months ago and it has just gotten worse. How did I do that? I told him that I only lift 2lbs weights. He suggested various stuff. Could I have done it while trying to get Adam out of his car seat, which is the biggest hassle and strain? Then I realized I carry around a 20lb kid everyday. So needless to say that I am not having a heart attack but I am still really sore. He told me to lay off the 2lbs weights which I think is really funny and to take ibuprofen & prescribed a muscle relaxer, which by the way kicked my butt yesterday. So no taking a muscle relaxer & going to work or maybe I will, wouldn't that be funny? : )