You may hate to hear it but speaking from the same position, you're probably not naturally meant to weigh that much. I weighed about 10.9/10st and by eating pasta pretty much every day I now weigh nearly 12. It happened pretty quick but before that I found it near impossible to gain any weight because of my natural motabalism, and the fact is if I worked out super hard, most of the weight I gained would just drop straight off me again and I'd just be skinny and ripped. If you eat too much now you'll slow your motabalism down and end up being fat in your mid 20s unless you keep working out super hard constantly.
I would advise chicken, pasta and protein shakes every time you do some exercise. Make sure you keep eating loads of fruit and veg too though.
If you're getting stronger, then I don't see whats wrong with weighing less. Its an advantage to be stronger and lighter.
NYBA Champ @ 170!