Assuming "clockwork" doesn't mean that it happened twice,
Most people will gain several pounds on a carb up day, so that's weird.
I have known people who lost weight and tried to eat more carbs and kept losing and did find out that they had diabetes, and were peeing out all of their carbs that were spilling over, and getting dangerously dehydrated in the process.
You can get a container of ketostix and see if you have ketones. You should not have any detectable ketones in the morning after a high carb day.