It could also be, simply, that you went out too fast. (I don't know how intense 17-18 is for you... might not be.) However, could also be that a shot of GU ... or OJ ... or a banana... after half an hour and you'd have been coming in strong.
I've also noticed that dehydration can peg me out... I'll feel stronger after I drink plain ol' water. (I don't eat much on rides, myself.)