X-ray on back showed no problems...?

I have been having serious back pain for the past month and a half or so. I did not fall or anything, but I am a dancer and some of the moves put a lot of strain on your back. I thought that the pain would go away after a week or two of rest, but when it did not, I decided to go to the doctor. I went to an orthropedic doctor. He took x-rays and said that nothing showed up. He said that the discs looked fine and everything was in line. I have a slight curvature, but he said it was nothing to worry about. Basically he said that it was most likely just inflammed and should resolve itself and take pain killers in the mean time. However, I am still in constant pain. It is affecting my everyday life. My question is, what are some back problems that would NOT show up on a traditional x-ray? (he did not do an MRI or a CTscan) I am aware that I tend to slouch a lot when I am not dancing...maybe bad posture could be contributing?