Okay, so I did it again today, guess what.
It happened again. Why?!?!
To eliminate the possibility of hidden skeles shooting arrows, go into Peaceful mode and try jumping into your waterfall again. If nothing happens, it must have been a hidden mob. If the same thing happens, it must be related to the blocks in the waterfall and you can start breaking things up to see what it might be.
I've tried breaking the surrounding blocks today, and I tried again, and this time, it didn't hurt me as I got to the bottom. Thanks for the help, no worrying that I might die when I'm on low heart. Thanks again!
To eliminate the possibility of hidden skeles shooting arrows, go into Peaceful mode and try jumping into your waterfall again. If nothing happens, it must have been a hidden mob. If the same thing happens, it must be related to the blocks in the waterfall and you can start breaking things up to see what it might be.