Rat limping without any injury

One of my rats spent about two weeks not touching one foot on the ground. His foot didn't have any injury or signs of something that could cause it and he didn't seem to be in pain when I touched it. A few days ago he just started walking normally again. I know I probably don't need to worry since this is not bothering him anymore, but I'm curious to know what could have caused it.

He could have sprained his leg or have had some kind of inflammation to it. Remember, rats are prey animals and will not show pain, no matter if you are touching it. This is why it's hard to determine when a rat is not feeling well and when we really notice it, it's usually far along.