How do I gain weight fast?

i got out of a long term relationship. i knew it was unhealthy and my ex finally left me alone which is great! so during the relationship i was always stressed out which affected my eating habits, i dropped a ton of weight at least 12 pounds and i'm very petite, i have never weighed 100 pounds.

i am scared because if i force my self to eat i sometimes throw up a few hours after eating. i try eating more but get horrible stomatch cramps.

any tips on how i can gain it back quickly? or any tips on what can get my apetite back? keep in mind i have already consulted a doctor and they have no solution besides eating more!
How do I gain weight fast?
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