The headache and heaviness aggravated by studying can be due many reasons. Assuming that you are a student, it is unlikely that you have a refractory error (near vision problems are usually seen in middle age). Association with studies almost rules out other causes like sinusitis
When mind is forced to do what it does not like, it will react in such a way that you have difficulty doing it. Headache, decreased concentration and forgetfulness are some of the problems commonly seen in students. High expectation from self and others only add to the burden. Following points may help you overcome this problem:
1. Take frequent breaks in between study sessions
2. Make the Studying process interesting (like group discussion, audio-visual aids
3. Take help for difficult subjects(coaching/tuitions)
4. Plan well to manage time. Remember, "Failing to plan is planning to fail"