As per the information provided by you ,it appears that you are sitting for long hours for study, in such cases there are chances of getting deep vein thrombosis which can cause symptoms mentioned by you. Especially if you are over weight too. It is rare in your age but it will be better to rule out DVT
Next Steps
do Doppler studies of the swollen calf
Health Tips
avoid prolonged sitting . have Ginger and black pepper with honey.
best wishes for exam