Oh Move you have my sympathy. Have you had a fall? Could it be a cracked coccyx? If not, might it be a slipped disc?
A few years back I thought I had a pulled muscle in my posterior. Then I started to get mild sciatic pains. I mentioned it in passing to my doctor, who was also a friend, and he tested my heel for a reflex, there was none and he diagnosed a slipped disc. Within a few weeks I was suffering a lot of achy pain in my lower back which echoed around my stomach. I couldn't sit or stand for more than a few minutes at a time and I was walking with a stick as my left leg lost impetus. The only relief was laying flat. After a six months I could barely walk, was on large doses of pain killers and spent most of my time laying flat on my stomach on a put-you-up in the living room as I couldn't climb the stairs. I had a discectomy which fortunately worked and I'm fine now apart from a few numb patches on my leg. I'm so glad my doctor got me into the system early before too much nerve damage was done.
Get thee to a doctor!! Tomorrow!!