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.
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.
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 :)