Hello sir. im a student and i feel strange heartbeat like pain in the top left backside of my head. it all started from november 2020 when i lost 20 kgs in a month with some tough exercises and diet. it was in neck in its early stages but now its all?

Neck Pain, like the one you have can be caused by muscle sprain(injury) from overuse of the muscle. Such pain subsides with muscle rest and use of mild pain medicines like Tylenol(Paracetamol) once every morning(may repeat once, 6 hours later.) If this does not control your pain in 2 weeks, then you need to see a doctor for physical examination/Tests.