What CY4NiDE said is spot on, "Best share" on CKPool is a per-round stat, not a lifetime score.
CKPool is basically treating everyone as one big solo miner. Each time the pool actually finds a block, that round is over and a new one starts. When that happens, your "Best share" gets cleared and starts counting again from the next shares you submit.
Seeing it jump from a few G down to hundreds of M doesn't mean anything broke or restarted in a bad way, it just means a block was found and the scoreboard for that round got wiped.
I have now became 31st on the CKPOOL top 100 user difficulties since this post lol CRAZY