Does it feel like muchos pressure on your ribs down near the bottom of the sternum where they meet?
I have a chronically confused hypothalamus, so oodles of weird symptoms as a result which i shant go into, but I've noticed this pressure feeling recently and since you mentioned the hypo' I figure it might be related.
If it is similar, it feels a bit like a muscle cramp and happens mostly in the evenings [right now as it happens] when I'm lying down or half way between sitting and lying. If any of those muscles are slightly tensed I make a point of relaxing them completely and it seems to go away after an hour or two at most.