<human>: Can you provide a brief overview of the text?

About 8 months ago, I got a prescription for mirtazapine 30mg, mainly to help with sleeping problems. I am already on citalopram (also 30mg) for depression. 
Now I noticed that I have gained a lot of weight over the past 4 months.

***A little bit about myself***

I have had an almost stable weight for the last decade or so. 
Until I hit puberty, I was quite thin. People even asked me whether I had anorexia now and then. Truth is, I could eat anything and not gain a pound. Cake covered in whipped cream? Lots of chocolate? Not a problem at all. Then I hit puberty and first actually started gaining a little weight in my life. 
I'm 1,76m and weighed 64 kg when I was 17. (5' 9", 141 lbs). 
I'm now almost 31 and my weight has been around 66-68 kg over the last 10 years. 

I hadn't been on the scale in a while, but got worried when my pants didn't seem to fit anymore...when I checked my weight again 2 weeks ago it hit me....75kg  (165lbs).  

Now I haven't been eating very healthy in the last part of 2014, but I'm really not sure how that could have been such a major factor. I've never been a healthy eater and I've never done any fitness or sports.

***Now to my actual question*** - is my body weight just a simple calculation of calories/unhealthy food => body fat? 
Or does the medicine actually influence on how my body converts calories to fat. 

Mirtazapine is known to have weight gain as a side effect, but I've heard that's just because it makes you want to eat more. 
I'm now thinking to stop with mirtazapine (although I feel it has had a very positive effect on my sleeping problems), but I'm just deeply unhappy with my weight gain. 

For the last 2 weeks, I've cut down on carbs and sugars a lot, but until now, I haven't lost any weight at all...  :(
<bot>: Same weight for the last decade. New medication, suddenly more than 10% weight gain. Is it just because I'm eating more or do the pills influence my metabolism?