Hello,
Adhd is different from being unable to focus. More often as you have mentioned that you probably required very few hours of dedicated attention over to your studies which have been your habit patterns. Probably this might be working at your back end.
When you say that you are appearing for a competitive examination these would also have made you enroll to a training centres' meaning a lot of mock exams would have been conducted in the form of practice tests so due to that to you would have been able to perceive the progress sheet of your studies.
At 22 there are many reasons for a person to get distracted one needs to know what is the leading causes of getting distractable if it's particularly while making attempts to your competitive examination which would determine the future prospects of the career you are about to choose post it.
Next Steps
if you are still unable to understand the cause of / reasons to your inability to pay attention during your study hours do go in person and meet the psychologist who would be able to assist you with.
Health Tips
Apart from that you could try practicing the stuff which you might have used it before during UG level or at times during the primary educational program.