Your brain needs food.
"Cookie, om, nom, nom!" – Cookie Monster
Seriously, did you know working your mind hard means you burn even more energy?
And that effect is made even more significant when you're tense or anxious as all of that adrenaline means your body takes over at a higher rate.
Studies looking at Chess Grandmasters show that during their hardest tournaments they can actually lose weight because of all this brain-buzzing activity. In fact, one expert estimated they could burn as much as 6,000 calories in a day without even leaving their chair!
Okay, so you might not get through that much but it does explain when you get the munchies while you're studying.
You need to feed your brain, and as your brain uses, basically glucose as fuel what you will crave is sweet stuff. So think fruit, grapes, blueberries, things you can eat plenty of.
Most of all, try not to worry. If you eat a bit more and a bit different in the seasons of exams, this could be why! Try not to worry and remember it isn't forever and things will settle down once you get back to a normal rhythm.