There have been different side effects for different mood stabilizers. Some antidepressants cause weight gain while others cause weight loss, some cause hair loss, some cause even more elevated mood. Definitely ask your doctor though if you are concerned about it so you might be able to get on a different kind.
Some anti depressants do cause people to gain weight, and a lot of people on this site have posted that they have gained weight whilst taking anti depressants. I myself have gained a lot of weight. I think there may be some anti depressants that do not cause weight gain. But we all react differently to drugs. Some people gain weight some people lose weight.
You will probably find, that in your case, that it is the anti depressant medication that is causing weight gain. It has been said many times too, when people post about this, that it is better to control your depression than worry about weight gain. it is of course, a personal thing.
Hopefully now you know the cause of your weight gain is the medication, you will not worry about it, and you have to try and eat sensibly.