What is going on?

Basically my brother sent me a message saying "wtf" and when I opened the message the first text was there. I didn't get a notification for it, it doesn't use the alt text bubble and he swears he didn't send it. On his phone, he has a message from me saying "rare itchy nuttsack W" which doesn't even appear on my side, have we been hacked or is this some kind of exploit or glitch?