Slowed down video

My camera roll slowed down a 55 second long video i sent to my friend to show off my new cd player. When i sent it i was not in slow-mo, as the original video in our messages is still fine and not corrupt. A few days later, i went back and looked at it in my camera roll, but it was slowed down after the first few seconds with flashing lights, and made my video over 5 minutes long. I held down on the video and it gave me the option “convert to original” but that did not do anything. After that, i tried to save the old video from the original messages, but it was the same as the corrupt one, even though in my and my friend’s messages it played fine. I then tried to copy and paste it to send to someone else, but when i went to paste it into a different chat it froze my phone and i had to restart it. I tried to copy and paste again because i thought it was just a fluke, but it happened two more times, the exact same way. This video doesn't mean a lot to me so it’s not a big deal, but i thought this was an odd experience and want to know why it happened.