I’m like really confused right now

So my boyfriends name on messenger changed to ‘fat boy’ just on the messages between him and I, not a group chat or anything and when I seen it I was confused because he hates being called that as he as recently gained a lot of weight and isn’t used to being big (I will always love him either way) but I’m just fucking confused because he say he didn’t changed it? And I didn’t? So wtf is he lying? I don’t see why he would do that in the first place or do you think he might be getting hacked? Idk I’m just asking cause I’m confused asf