If it is at the bottom of your left rib, it could be an issue with your liver, not heart.
Pretty hard to tell from your description but what i would first look for is the position of the pain...remember that the heart sits dead center of our chests between the nipple lines and very very slightly to the left and is about the size of your left fist. So if this pain isn't in that general area it could very well be muscle related. I always do the touch test and wherever i have a random weird pain i take my index finger and press it to the area that is feeling weird and if its an "ouch" moment i know its muscle related because you can;t touch your heart thru your ribs. It could be the way you are sleeping, could be the work that you do each day...i always say when in doubt call the doc.............