What Causes Throbbing Ache On Inside Of The Top Of Arm/shoulder Blade/breast?

For the past couple of weeks I have had a dull throbbing ache on the inside of the top of my left arm, which migrates to my left shoulder blade, left ribs and across my left breast on occasion. I’ve also been woken a couple of times in the night almost feeling like I have a stitch. This goes off if I move. If I keep warm and still, it seems to gradually alievate to the point of being forgotten, then as soon as I get too cold / start shivering for any length of time, it all starts up again.

Logic suggests this is some kind of muscle / ligament pull which gets aggrevated when I get cold but given it’s location I’m making myself sick with worry that it could be something far worse!