Hi bella,
I'm around the same height as you and for a long time I was 7.5 stone, it was only really about 4 years ago I started putting weight on! (I'm 22 now).
Even now I have put weight on and got up to around 9.5 stone, I go through phases of just not being interested in food, and same as you, people I work with and friends nag me to eat and always say I dont seem to eat much. But then I also go through phases where I can eat like a horse and munch on everything in sight, (but people dont seem to notice those times!!)