I had a feeling. I went through the same thing a year ago when I went on the keto diet. All of the sudden, short, easy bike rides were kicking my ass.
It's a tough transition for a sugar burning cyclist to become fat adapted. When not eating low carb and cycling, your body gets used to a steady intake of sugar to burn for bike rides and doesn't become efficient at metabolizing fat for energy. When you make the switch to low carb, your body has to literally change its internal systems to better metabolize fat. That process is a bit slow.
So when you go out and try to do your same rides on lower carbs, its going to suck. Bad. The good news is this will pass. It took a few months of eating low carb and slowly increasing my cycling back up to pre low carb levels. But I eventually got even faster on the bike than I was before I went low carb. Another benefit is once you build the internal machinery for efficient fat burning, you essentially become bonk proof. You can ride for hours in a fasted state with no ill effects.
Bottom line, it sucks to get fat-adapted as a cyclist, but once you are there, its much better than being a sugar burner. Just take it slow and feel your way through the process.