There are many things that can be wrong. Pain in your back at night is an actual medical problem call night back pain, ironic isn't it? It can be caused by many different things like degenerative disc to a sprain. It would probably be worth it for you to go to the Dr and have it checked to make sure it is nothing serious. It can make life miserable :( Best wishes, Sissie
How Long has this been going on? Do you have any other symptoms? Back and side pain can also be a sign of urinary Tract or kidney infections. You should definitely pay a visit to your md. If it is an infection you don't want it to get worse. I hope you feel better soon!
It might be your mattress. Try sleeping on the couch, or in a different bed if you can, and see if your symptoms continue. I think it's recommended that you get a new mattress every 8 years, or something like that. It's worth a try! Good Luck,