How can I gain some weight?

I've been vegetarian for 3 years and have never had any problems health-wise. However, I went vegan a couple of months ago and have lost a considerable amount of weight. Well, according to the scale, I've only lost about 5 pounds, but my bones are sticking out all over the place and I haven't been getting my period. =S l've always been kind of bony, but not to this extent. How can I gain some weight on a vegan diet? I'm 14, by the way.