yes i totally agree with wannabenana because there are people who eat and eat and eat and cant gain weight and it might be because of something else and not just a really overactive metabloism...so GET A PHYSICAL!!! good luck!
Make an appointment to go see your doctor and get a good physical and blood work up to rule out any underlying problem you may have.
i am 5 foot 10 inches tall and i know the weight is too less in accordance thats why im really worried!
how tall are you? i converted 51 Kg to pounds and it tells me you are 112 or so pounds...now if your like 5 foot id say thats fine, but if your like 5 foot 9 thats way too skinny. so im going to have to know how tall you are in order to give you accurate advice.