Left arm upper area(around tricep muscle) hurts now and then.?

I have no idea why but sometimes my left arm upper area muscle hurts. It stops and then starts again sometimes. It might happen once a day or sometimes not until a few days.
Update: I've never used it to excerise, I get loads of calcium, but that sleeping on it might be correct, but i do sleep on my side both ways.
Update 2: Wow it could be a heart attack?Are you serious?Wow now I'm frightened, I'm only 17 and I sure as **** don't want that to screw my life up.
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