I would like to know if there are no active lesions in the brain, how come my left leg is getting worse?
Demyelinating events anywhere along the circuit between the cortex (surface of the brain) through the brainstem and in the spinal cord can lead to weakness in the leg. A stable MRI of the brain doesn't really help answer the question you are asking. Your neurologist should be able to help determine if a spinal cord event has occurred which can also cause weakness.
Another possibility is that the MRI is not sensitive enough to see the demyelinating lesion to explain the problem. As sensitive as the MRI is, it is not perfect.
Yet another possibility is no new lesion (or MS plaque), but degeneration of the circuit due to prior inflammation. This can occur in what is known as secondary progressive (or progressive) MS.
This answer is assuming that MS is to blame for the weakness. You doctor can also evaluate for other potential causes (if appropriate) given your situation.
A. Scott Nielsen MD MMSc
Neurologist and MS Specialist at Kaiser Permanente