What will happen if you go to gym but don't eat enough after?
You have to understand how human body works. When you are in gym you exhaust muscles and only afterwards they start repairing/building process ( kind of like the more you exhaust and fuel with food, the more they grow ). Our tissues are made of cells made of proteins (diff kinds- some can be produced by body and some has to be taken from food ) which is made of amino acids. If you don't have sufficient intake of nutrients, your muscles will never build back esp after training. You might be happy about your weight drop on the scale but it's not fats, it's definitely muscles.
Never skip a meal after workout. Preferably within one hour. It doesn't have to contain high sugar levels or carbs, but it has to be rich in protein. Sugar and carbs main portions are best taken before training to provide you with energy needed. Try this and you will be amazed by results - performance in training and muscle growth.