creinha,
The stiffness you mention in the morning is not indicative of a herniated or bulging disc - that is much more likely to be from some type of arthritis or inflammatory problem.
It is quite possible that you have some degeneration somewhere in the spine (possibly arthritic in nature) or muscle strain or weakness problems. With the nerves in the spine and that exit the spine, a very small problem can equal a great deal of pain.
Also, I have had several spine MRI's and I am amazed at how different doctors interpret the same MRI so differently. What one doctor calls mild or doesn't even mention another doctor will call moderate or severe, so a lot of it is left to interpretation.
You might want to talk to your GP about exercises or physical therapy you can do to strengthen your back - these can help just about any back problem you may have. My doctor is a big advocate of pilates for the back. Also, if you are not already taking some anti-inflammatories you might want to check with your doctor about that.
If you've tried everything and are still having problems after a few months, it would be good to get a 2nd opinion.
Good luck!