Are you taking Gabapentin ? My missus has CFS/ME + Fibromyalgia and was getting the same headaches you describe, lasting from all day to several hours at a time, the pain was like a woodpecker constantly tapping on the inside of her skull, over the left side, predominately. She had been on Gabapentin for over 3 years to help her sleep at night cos of the pain from everything, I convinced her to slowly withdraw from it and after several weeks of bad headaches and other withdrawal symptoms, the headaches have stopped and she hasn't had one for over a month, She also cut down her Paracetamol intake from 8 to 2 a day, I was concerned for her Liver too as its very easy to OD on Paracetamol and she's been doing 8 or more per day for 3-4 years. Not saying if your taking Gabapentin you should stop, but might be something to look into.