If you live in a developed nation, it's very unlikely with modernized medicine. I was underweight with my first. You need to talk with a doctor and find out how much weight they want you to gain, then try to meet the goal. This is for the best outcome for both baby and you. Remember baby needs you to eat. If you have a medical reason why weight gain is hard, it's even more important for you to see a doctor asap.
No but you can always ask your doctor their answers will be best certified