what could be the possible reason for losing muscle mass when exercising?!

The body will always use these in the following order; Carbohydrates, Fats and Protein. If you have no carbohydrate stores, fat will be converted to energy as fat has more energy per gram than protein. Once the fat stores run out, the body will essentially 'eat' itself hence the loss of muscle mass. Vastly increasing the amount you eat will stop this. You could however have a metabolic disorder, so go to your doctor to get some tests done.

Candie is right; lack of dietary protein can make a normal amount of exercise affect you like 'overtraining' does.

Overtraining can also cause you to lose muscle instead of gaining.

A low-energy diet will cause you to lose muscle mass, whether or not you are training.

(Protein is removed from muscles to make you weaker, as a way of conserving energy.)

The most common cause of muscle loss while training is the wrong TYPE of training.

Working HEAVY (getting exhausted quickly, so that you can count the repetitions instead of measuring the duration of the exercise) is what stimulates muscle growth. You can use a little of this type of exercise to maintain muscle mass, or perform several very short sets in a session to make your muscles bulk up. This process is called 'hypertrophy'.

Working slowly can also stimulate more muscle growth, IF you keep 'perfect form' and lift heavily enough.

Working light can still make you stronger, while causing your muscles to get thinner and more dense. If you perform an exercise for as many repetitions as you can do in a row, while always trying to add repetitions or complete the set faster, this will increase your ability without adding bulk.

Food intake affects muscle mass greatly. Without something for the body to burn while exercising, it will evantually use up muscle fibers instead of the fat cells.

maybe it wasnt muscle but fat.

not enough protein