How do I more gain weight?

Today I went to the doctor and they mentioned to me my weight has been dropping even though I'm still growing (I'm 18). It's probably due to the fact that I'm not playing sports any more and eating as much as I used to. I'm also working after school frequently so I often forget about having dinner. I'm a vegetarian for many reasons so I can't eat meat or fish. How do I gain 5-10 lbs in a month or so?
How do I more gain weight?
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