You might be eating less, the calories you are taking is you burn all at gym.
Try to eat more, eat protein rich food, whole grains,cheese, fish, eggs, milk etc.
You should also try some weigh protein to add in your diet. It's really good in gaining mass. Also consult your PT because there are some workouts that are designed for cutting down muscles and there are some that is designed to build muscles. Maybe the one you are doing is to toned down your muscles. Try eating more protein and also take some multivitamins. Drink milk every night before you sleep.