Right, imagine this:
You manage to walk into a door or someone hits you with something on the arm/leg.
Initially you are in pain.
Then, after a short while (maybe a minute of two) the initial pain goes away and you feel alright again.
Now, imagine you're playing football and you get clattered in the leg by someone, and you're running.
You fall over, you're in a bit of pain.
Then, after a minute or so the pain goes away and you can continue on.
Of course, I don't know about everyone, but when I get hit in some way, it hurts quite a lot to start with, and then kind of goes away and I feel OK again shortly thereafter. Just like what appears to happen to football players, they go down and then magically feel OK a short while later.
Strange, no?