Being to under weight to get pregnant?!

lilliaunasmommy • Have 2 dd one 3 year old and one 7 month old!. Im pro mind your own business and stay out of others. 😉
Soo, I have a question! I've been severely under weight the last four years. The doctors don't know why. I have a one year old daughter. I weighed 100lbs. When I got pregnant with her. Me and my s.o. have been talking about having another. But I currently only weigh 90lbs. Could this effect me getting pregnant? If so does anyone know of ways to gain weight?! Thank you in advance! 😊