raz,
I'm no doctor, and don't know every detail of your position, but I'll offer my two cents. Overtraining is always a possibility, but it sounds like you've been reasonable with your build up. I'd get a full blood work done if you haven't - that never hurts anyway. One thing did stick out. You are 5'7" and 112? Is that right? That is CRAZY skinny. Elite runners aren't nearly that light. For reference, I'm your size and weigh 20 pounds more, and I run competitively. I think you gotta put on some mass. I don't know what that comes out to on the BMI scale, but I'm sure its unequivocally underweight. Low body weight can hurt performance just as much as high body weight. Eat up. Don't think about it, just eat what you feel like. The doc I'm sure will also be able to shed some light on things. Keep us posted.