What Causes Irregular Periods While On Strict Vegetarian Diet?

I am a nineteen year old strict vegetarian. My period has been inconsistent since I became a vegetarian. I have gotten in these moods where I just want to continue eating until I start my period because gaining wait does help. I would like to stay on the thin side for my frame but would also like to have a healthy cycle. Can you recomend anything. I have gone two months without menstrating and when I do it is extremely light. I feel as though I have not had a healthy full period since I have been thin and vegetarian. When I gained weight and was vegetarian I did have normal periods but I have lost the weight again. I used to have normal cycles as a meat eater when I was even a bit thinner. What can you recomend? I am a young healthy girl and I feel as though having a normal cycle is a major importance.