Underweight but i'm not anorexia! Why am i not gaining weight?

I am considered severely underweight by the doctors and dietician since i came back from my transplant (weighing only 29.5Kg). My god, i think this is my weight when i was in Primary School. Well, I lost about 6kg after the surgery and given my dry weight(35Kg) during dialysis is already very low, its a no wonder that i'm severely underweight.

I thought i will gain back my weight pretty fast given the good food and nutrition that mum was feeding me, but apparently it was not so. After 3 months, i have only put on about 2kg. I remembered the dietician asking me in the hospital whether i am suffering from anorexia, what a joke, i love to eat, how can i be suffering from anorexia.

The slow weight gain didn't bother me much untill i realized the problems it brought. When i was hospitalised for CMV, the doctors said my immunity is very low, partly due to the immunosuppresant i'm taking and also because of my small body size. Then as i did more research on the internet, i found that low blood pressure and irregular menstruation could also be due to loss of weight. Furthermore, i could feel that my stamina is not as good as before and body being weaker.

Ok, so the crucial thing now is to gain back my weight and be more healthy. According to my research on the internet, in order to gain weight, one need to consume more calories. The dietician asked me to eat oil (good oil like olive oil & seasame oil) to gain calories. Sounds oily, so i didn't really follow his advice. Well, maybe the reason why i'm gaining weight so slowly is because i have been following a good diet, low sugar, low salt, low cholesterol, less oil, therefore low calories. Haha...