why does my low back hurt when i lay down?!

It sounds like you may have a herniated disc in your lower back. This is exactly how I felt when I first found out I had one. No amount of stretching helps as suggested by the other contributor who offers this same advice for everything from sore throats to headaches.
You will notice your back feels better after moving around some in the morning but hurts much worse when laying flat. Putting the pillows under your knees is a good idea as it does take some of the pressure off your lower spine. Generally the herniated disc is in the lower lumbar area.
The best thing for you to do at this point is to go see your doctor and have some tests done. If it is a herniated disc he or she can give you something for pain and may refer you to physical therapy to strengthen your back muscles or possibly relieve pressure on the disc and spinal cord. Until then, try not to carry anything that is over 20 lbs. Don't bend from the waist to pick anything up off the floor or a table. This will only strain your back more I replaced my mattress but to be honest it really didn't help me very much. Chiropractors have their place in medical care but, this is not the time for them. You need to see your doctor and if his tests confirm you have a slipped or herniated disc he or she will refer you to a neurologist or a neurosurgeon for further evaluation if necessary. These sometimes heal themselves with proper treatment. Go see your doctor.

Nurse for 30 years. 2 Herniated discs in back.

You have pinched back muscles causing that pain. Going to the chiropractor helped put them back into place but as they were still tight it was easy for them to be pulled back out of place and back into pain. When you laid down you changed the pressure points in your back to cause more pain. When you sat up the pressure points hadn't changed much so the pain didn't set in as much. to get rid of your back pain and to keep it from coming back you have to free up your back muscles and here's how to do that:
Back:
(do from a sitting position)
Place your left hand on your left leg next to your body. Place your right hand over your left shoulder, fingers over the back and the palm in the front and firmly pull down on them and hold. After 30 seconds slowly lower your body forward and to the outside of your left leg, keeping your left arm fairly straight as you do. When you reach your lap remain there for another 10 seconds, release the pressure but rest there for another 30 seconds. Then reverse your hand positions and do your right side.
For best results relax your body first by taking a deep breath and exhaling then remain this relaxed.