My sister 18 has a neck pain & headache in the back of head for 4 months. She usually took painkillers but for past week seems worsened. Why?

Good question. The history is scant so anything may cause her headaches. Painkillers are not the answer for headaches. She needs to see a physician to determine the source of her pain. It could be a migraine, a muscle tension, a reaction to chronic use of painkillers, a food sensitivity or even a brain tumor, although that is rare.
Neck arthritis. There are several joints and intervertebral discs in the neck. These can wear out over time leading to pain. Degeneration of the joints or discs can cause neck pain, headaches, and shoulder pain.