Hmmm, kind of hard to really say with absolute certainty but it seems to be muscle-related.
I know that people can get muscle aches for no reason sometimes, I don't how that happens but it can occur. You haven't done anything to stress it or anything like that?
may be a muscle, but i have done nothing to hurt it and it is just in that on spot about 2 or 3 fingertips wide.
How deep is the pain, does it feel like it is just under your muscle? Is it right under the skin maybe?
It hurts when you push on it, which wouldn't really indicate a vein or artery as far as I know, I would be more inclined to say it has something to do with your rib or muscle, I obviously don't know for sure but I don't think pressing on a vein would cause any pain as far as I know. Usually if its muscular in nature, pain can be elicited by pressing on it. There have been some occasions where I'm laying on my arm when I sleep and I get an intermittent ache on the left side of my chest, but it feels like it is just under my skin, or in my muscle. When you apply pressure to it, does the pain stay localized in one area or does it radiate? Any dizziness or lightheadedness when it happens?
Of course if you keep pressing on it you will probably irritate it if it is muscle-related.