Yo ho Guest,
You're about a stone underweight, but if you're healthy & eat regularly, don't worry about it too much. Height and weight depends on genetics, as well as lifestyle, so look at your parents and members of your family before jumping to any conclusions.
If you're concerned, are not eating properly or have other symptoms (such as, not sleeping properly, feel tired all the time, etc) then go and see your doctor a.s.a.p.
Hope that helps.
Also- I forgot- you're still growing, so you may have another growth spurt and put on weight. But my previous comments still hold- if you're concerned or there are other things going on- please visit your doctor.