Try eating more often. You should be eating small meals 5 to 6 times a day. You should be eating something about every 3 hours. It will help keep your energy up. If 8 stones equals 112 lbs then yes you are on the thin side. I am the same height (5'7"), I think my best weight is 150lbs and I have a small body frame.

It will also depend on your age and body makeup. I am probably older than you, so a heavier weight would be better for me.