I was always being nagged by mum about my weight as I have always been tall and slim but then lost weight after, like you, going through stressful times and found it impossible to gain weight again. I went to the doctors who did a blood test, testing my thyroids mainly, but chekcing other stuff out too. All came back just fine and was told I didnt have to worry unless I had a dramamtic weight loss.
If you are really worried about why you cant put it on, go see your docs, if nothing else it will put your mind at ease that all is ok. Other than that, just eat three healthy meals a day and add a pudding to your evening meal. Im sure if you eat a varied diet and include all the food groups you will gradually start gaining weight but it will probably be very slow and gradual.