I have caused a malfunction like that with my Canon 600EX RT flash. For some reason, the flash is over exposing my dog portraits that I have been shooting in this way for months with perfect results. I know that it is the flash because I tried it on both my 7DII and my 5DII and both cameras/using the same lens, over exposed. Then I put my 420EX flash on my 7D and nailed the exposure. Nailed it again with the 5DII and 420EX.
I don't have the energy to trouble shoot the flash tonight. I will do that in the morning. I am sure that my problem is akin to Brian's. I just touched the wrong switch or button...