How old is he? How long have you had him?
Obviously he got stressed out. Jumping or falling is commun for chameleons when they feel threatened.
Now, what happened? I sincerely don't know and this is the hardest part of this puzzle. However, offhand, here's a few reasons that could explain it, at least partially:
-The speed you moved the branch (and him) was too fast
-He didn't notice you when you arrived (low chance that it happened, but who knows)
-The branch that he was on was too thin and he accidentaly fell
-The cloths you wore had colors that he feared (this happens sometimes...)
-An animal or something moving very fast toward it scared him, even if it was very fast away
-He noticed another chameleon close (if you have more than one)
-His health isn't very good and doesn't have a strong grip anymore (i sure hope this is not the reason, but the chances are very low since he is still somehow scared of you)
It's 3:38am so i may be wrong, who knows! But this is what i thought first when i read your post. However, i would like to point out to the fact that 1) you were cleaning his cage, 2) you took the time to come here and try to get more information, and 3) you keep an eye on what is normal and abnormal for your pet. So no, you aren't paranoid and you have great qualities and i hope you will keep them!
However, you mentionned that he was sleeping "tonight" in some water... This is something i don't really like. Sleeping in some water is not something i have seen nor read before! So, was he sleeping with the lights on? He was awake for how long when he fell asleep? Is it possible that he got hurt when falling 4 feet, based on his behaviors during the last 3 days? How hot is your enclosure during the night and when he fell asleep?
I don't know what others think, but i would suggest at least moving him from the puddle of water, since water can drain all his heat, even if he's sleeping.
