Sometimes I have to put my clients in a calorie deficit that is too steep in order to keep the fat loss progression going. The signs are usually a change in mood, lack of energy, headaches, hunger pangs.
It's at this point that I slowly "Reverse the diet." Every week we add more calories back to their diet in the form of carbs and fats. I'll generally at 50-100 calories per day, 1 week at a time.
Depending on the individual a person can actually continue losing body fat while reverse dieting. Sometimes NOT! This is because the deficit is steep enough that they remain below maintenance calories for a few weeks and the reintroduced calories are better partitioned to muscle growth and energy expenditure!
If you're in a steep calorie deficit and experiencing symptoms, try to reverse diet into a reasonable calorie level before resuming a "dieting phase"