Soreness versus Injury

I probably ask this question too many times, but how do you tell the difference between muscle fatigue/sorness and the initial onset of an injury?

Sure, I have been sore before - countless times, but over the past few weeks I have been experiencing some spasming, "hollowness," and tightness along my left shoulder blade, over my back, down my bicep, and even in my forearms and fingers. As I felt my back tonight, I could feel a huge knot that would actually pull on my forearms when I pressed down on it. I realize everything is connected, so it makes sense, but is this sensation the beginning of an injury? Is it just that I need a massage? Is it just that I need a rest day? Is it something to ignore and push through? Admittedly, it is negatively affecting my performance.

I think a lot of athletes struggle with this, especially when they are pushing toward the next level as I am. When is enough enough? When do you stop? It is not just a physical battle but a mental one as well. How do I keep this from discouraging me or getting in my head? This is drastically affecting my focus.

Always telling others to take a day off when something doesn't feel right, I feel like the biggest hypocrite in the world right now as I debate packing my bag for my WOD tomorrow. I don't want to let my 1 hour per day of working my butt off slip away tomorrow. I look forward to that one hour. But is pride standing in my way? Fear? Lack of knowledge?

How do you tell the difference between soreness and injury?