Under weight ?

I'm 5'5- 5'6 & I'm about 100 pounds ... Everyone has been giving me a hard time lately because of how skinny I am! I don't have control over it. I've been tall and thin for as long as I can remember! But I feel like there's health problems getting to me :( (weak, shortness of breathe, dizziness) I recently started taking B12 liquid and prenatal vitamins (I'm not preggo) just tl get the vitamins I need. I've been needing to see a doctor to get a healthy diet plan. I've been really busy so I need help! Any good, healthy , fatty, foods that will help?