Vegan and I can't put on weight?

I'm a 20 year old male 5'7 around 140-145 pounds. When I was around 17 years old I was the same height but I weighed 185ish, I was definitely overweight. I never changed anything though, my diet is similar and I don't workout very often. Now that i'm 140 I feel like a skeleton and it's worse than being fat. I saw a doctor and he said nothing seemed irregular just that my metabolism probably kicked in. I would love to be motivated to workout but with my picky vegan diet it limits a lot of the food I eat, and me staying small is not motivating. I eat BIG too though, I usually cook for just myself, when I eat pasta I will dump an entire box and half throw marina on top and literally eat all of it in one sitting. It fills me pretty good but I will get hungry again later that day....it's just insane how can I gain weight??