Why does my sternum feels bruised?

I believe I injured my sternum when I was a teenager. I had problems with pain for a while afterwards - and here's the weird part: sometimes if I sleep wrong, I will wake up with it hurting just a little, but then later it will 'crack' and it feels much better afterwards. My own personal theory I came up with is that since the sternum is one of the few bones that is actually a "fused" bone, containing smaller bones fused together, I must have hurt it just enough to loosen a couple of the fusion points, so that sometimes they can sort of go out of place. Don't know if this would be true for you, but...hope it feels better soon!