What could be the cause of increasing pain at the back of my thighs, started in august, sometimes fall back into bed or chair pain is so bad.?

Many causes possible: I do not have enough specific information to make an educated list of possible diagnoses. You need to see a doctor and have a carefil examination - perhaps blood tests, x-rays. Can start with clinic, or pcp but may need to see orthopedics or neurologist or other specialist. Dont't delay; would be best if you can go when you have an attack of pain.