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What are possible causes for my lower back and leg pain?

I'm 22 and since about a year ago, I've been having lower back pain that's been getting worst. I don't do sports and there was no injury to the area. It comes and goes but it's causing me more pain now and the pain is also in my legs. The pain is worst when I lie down at night and its a sharp pain radiating to a dull pain. The leg pain feels like nerve pain and it's been making my legs very weak and I have trouble walking at times. I also get this weird tingling sensation in my lower body and it makes my legs weak and knees buckle. I've been to several doctors and they told me it was muscle pain at first, told me to exercise, and gave me anti-inflammatory meds. None of that worked, and they are having xrays dones and my CT scan is schedule for monday. I'm pretty nervous, and the doctory said she does not know what it is without the test results and does not know what the problem could be. Anyone have similar experiences and can tell me what it was or any ideas what it could be?
Will CT scans always be helpful to find out problems relating to my back pain? Seems like some of you think that I should not be getting a CT scan. What is the worst possible problem that could be causing my pain? I just want to be prepared. I don't do any lifting or heavy exercise, so the slipped disc thing seems weird... Does it just happen?