I'm only 103 pounds and I'm 5 foot 7 inches

Do any of you know what I can do to gain weight. I know I probably sound crazy since people usually want to know how to lose weight. I have been very small my whole life and have never been able to just gain weight naturally. I don't work out. I don't go on bike rides or walks. I just do my normal everyday activities. I have been to a dr and asked for medicine to help me gain weight and was told no because they said in small because it's just how I am not because of any medical issues. I myself have low self esteem and am not comfortable in my own skin. I am very small and really would like to get some meet on my bones. I'm small chested and have no butt either because of this. My fiancé says he loves me just how I am but I don't like how I am I want to be thick not a skinny twig. Please refrain from making rude comments....