Very skinny and exercising?

[Alex]

I am extremely skinny for my height (I am 6ft 4 and weigh around about 67kg with a BMI around about 17-18) and have been following an exercise plan to get a bit fitter. I've been mainly working on upper body and I've noticed big changes in the size of my arms and my abdominal muscles are becoming more pronounced. However, at the same time, it looks as though my ribcage is becoming much more obvious and the bones around my sternum look much more obvious and I wonder whether I am using up all my fat reserves. I haven't really changed my eating habits and my weight has stayed pretty much the same. Is this alright? Or should I try and increase my eating to compensate for this extra exercise? I struggle to put on weight, regardless of what I eat and feel that although I eat less for guys my age, I am by no stretch of the imagination starving myself. It doesn't matter whether I eat really healthily or like McDonalds every day, there is no change in weight. Any advice?