Terrible headaches not caused by tumour

My Mum was diagnosed with lung cancer in December 2005, it has spread to the bone and there are secondaries in the lymph nodes. At one point there was a small tumour detected in the brain, but since she has been on Tarceva that has disappeared. Mum has been on Tarceva for over a year and it has` worked for her at keeping the cancer from spreading further, there has been very little change in tumour size in a year. However she has been experiencing terrible headaches` every 7-10 days which result in her being bed ridden, violently sick with diarrhoea  and all she can do is sleep. The headaches start with her having a muzzy head, being confused and slightly off balance. Usually after two days she is starting to feel better and then she i...