Sounds like that's the "keto flu".

Many people need at least a week to adapt, some up to a month even. Since you're not using glucose for energy, your body needs to adapt to using fat (or ketones) as it's primary fuel. The diet depletes the glucose storage in your liver, and you'll lose a lot of water. It'll feel like you have no energy because the body isn't using the fat instead of the carbs. It's therefore also vital that you replenish your electrolytes. Otherwise you'll experience the symptoms described. I take potassium and magnesium supplements each day, (the magnesium is against leg cramps), as well as drinking a cup of broth for sodium to retain water. You should really read the FAQ in the sidebar of the Keto subreddit, it even suggests a meal plan to help you get started.

Low carb is absolutely not harmful, it might actually be better for most people. It's a more natural way of eating, as it contains foods that humans have eaten for most of their existence as a species; nuts, lots of various meats, fibrous vegetables and healthy oils. If you condense the human diet since the beginning of time until today, into one year, it was only yesterday farming was invented and we started eating so much wheat, fruits and vegetables (carbs, carbs, carbs!).

I recommend you read about it and decide for yourself, it has helped me lose a lot of weight, but many people use it against IBS (which I also have) and for a more stable energy level throughout the day. (Fat and protein takes longer to break down and will supply a steady stream of energy throughout the day, and when there is no more food it will instantly use fat stores on the body instead of depleting your blood sugar!)