The normal/healthy amount to be thinking about it is very little/never.
The fact that you know (and potentially controlling?) roughly how much you're eating each day (which still isn't very much, especially given that you're doing exercise as well) is kind of a sign that something's not quite right, in fact the whole of the second paragraph (and bits of the first) suggest this.
From what you've described it seems like these thoughts might be starting to take over/control a lot of your life? The effects this can have, both mentally and physically, can be really serious. You won't get fat overnight, or even in a few weeks or months. I've had similar issues with food and exercise in the past and, although it took some time to realise and come to terms with it, I'd now much rather be a bit chunky and actually happy and healthy, than skinny, weak and miserable. Your body is the least interesting thing about you. You need food just to function, and food is so much more than just fuel; it's not something to be earned or made up for.
I'd really recommend speaking to a professional about this, before it escalates into something potentially more serious and controls your entire life.
I don't know if anyone has ever expressed their concern for you? They're not trying to make you fat or unfit or whatever; they're doing it because they genuinely care about you and want the best for you and your future.
I really hope things start to get sorted for you
