hai, underweight should be decided using Body mass index. but you have a past history of amoebiasis and ulcer possibility of weight loss occur. at present you didnt have burning sensation and other ulcer related problems still you should be careful in taking food. underweight is not a only symptom for amoebiasis. unhygienic food will lead to again amoebiasis. follow the advise : take hygienic food. take food in regular interval. dont skip food timings. First eat protein nourished foods like Almond,cashew,channa,soyabean,peanut in your daily food intake. take diary product like milk,yogurt,chesse,butter etc. Add banana and tuber vegetables. jaggery,black gram,and dal prepared with ghee. This will nourish in a healthy way and gain weight. Avoid taking hot,spicy,fried foodand caffeinated drinks. Hope I have answered your query. If you have any further questions I will be happy to help . For Follow up queries, end with If you do not have any clarifications, you can close the discussion and rate the answer. Wish you good health