Lower back ache and pelvic pains when I lay down.

I'm getting really bad lower back ache the moment I lay down, including twinges of pain in the middle of my pelvis below my belly button. It feels like a slash of a knife each time it does it. 
I was walking around and it was okay, but the minute I lay down it comes back really bad. 
What could this be? 
I'm not due on and I'm not ovulating. 
I've taken painkillers and have had a hot water bottle on my stomach for a good hour, yet nothing is helping. 
Help?