You are way too much stressed that is why this has been happening
Rather than stressing over this you should plan things in a systematic manner.
set up small goals and focus on yourself too.
Prepare schedules for your studies. Divide your time with breaks in between so that you need not feel burdened at length. Although it was long back, I cracked multiple medical entrances too. So I'd suggest you to study efficiently. It doesn't matter how long you study, what matters is how much you were able to grasp. As for you weight, don't skip meals. Follow a healthy diet which provides you nutrition required to study well. If the issues persist, it would be better to consult a physician. All the best!