Ok three possibilities I see:

1. Tsunade is uber good at healin, Lee is uber good at speed, so catching up to them is no problem.

2. Lee already had his sugery before the Genin team left and was just recovering at the time, even though he had his crutch and bandages on he probably doesn't rely on it as much. He decided that he was in good enough condition to help his friends.

3. Lee is still injured, he somehow got temporary boost in power (whether it be opening a gate or eating a couple of those soldier pills). Lee wants to protect Sakura from any pain, emotional or physical - even if its screwing up his chances of being a ninja again Lee came out to help.

Lee is lee, there is no reason why someone would do a henge (unless its a really stupid diabolical plan made by Orochimaru)