I started with a bad headache 6 days ago, then sickness, maybe because of the pain. A couple of days later, suddenly in the middle of the night I woke up with back pain across the sacrum area, spreading down the middle of the back of the thigh, both sides. I am unaware of any exercise/inury sustained recently. Once a very strong painkiller wears off, it all comes back: headache, nausea, back pain. What can it be?