Eating may not do it because you need to eat the HIGH calorie foods and you may not be. You could be a healthy eater and therefore it may be hard to gain weight.
You can try getting the drink called "Ensure" or even just the carnation instant breakfast. Anything to add more calories to your diet.
Your metabolic rate must be high but then you might be young and "geared" that way through your genetics "roadmap".
Why worry about it if you are healthy? You best advice for any health matter must come from your physician/nutritionalist.
I wish i new how to gain weight to im 6ft 5 18O i look like a damn skelton lol
Talk to a doctor about that. Some girls really can't gain weight (Iucky b****es, just kidding) so ask a doctor for recommendations. Eating a lot of crap won't help anything, but eating good things probably will.