They have only ever split shards one time. It was a fleet arena shard that was super unbalanced basically because it was the one that all the people who were super anxious for the fleet update and got in it right away after the mode came out.
They split this shard into several shards. This is the only time they've done anything like that.
They have said they will not mix up shards ever so you're pretty much stuck in your shard. You may have been in one of the ones I was talking about (that started as 1) which would explain the progression.
How long have you been playing @XaVii ?