I was only 105 before I got pregnant, I was never able to gain weight I barely got to 100 lbs after my first year of college. But I still didn't gain weight until I was around 17 weeks and it was only a few lbs once I reached 33 weeks I started gaining more weight and am around 125 so you'll be fine :)
My dr referred me to one because I couldn't take prenatal vitamins and it was well worth it. I only gained 11kgs and lost 8kgs at birth and baby is very healthy.
As long as you are taking prenatals and keeping a balanced diet you should be fine unless the dr tells you other wise. Every pregnancy is different. Some gain lots of weight some not much at all.