Mirtazapine (Anti depressant drug)?

I have been on mirtazapine for now 4 years. In that time I have put on about 16 kgs, which has been good because I was quite skinny. I have lost about 3 - 5 kgs in the last 4 months. I think maybe because for a while I was quite stressed, I don't go to the gym to do weights much more and in the last week I have been ill with the flu. Is the weight loss due to these things or is the drug starting to not have the same impact. I want to put those kg's back on. Any advice