Numbness - feet, legs, buttocks, waist--both sides. also upper left back. gets worse and "climbs" while sitting in car. if not ms relapse, then what?

Very well: Could be an ms relapse. Please call your neurologist and get evaluated.

With a history : Of ms it would be wise not to fool around and make an appt asap with your treating doc.

Numbness: If you already have been bothered by MS, certainly this is possible. Another condition that is sometimes worse with sitting is called radiculopathy. It is caused by irritation of the nerve roots in the spine. It often causes numbness in the legs and may change with position.

Comments: Not sure if you carry diagnosis of MS, but you could possess a problem affecting your spinal cord, or you might have a peripheral neuropathy. This can be easily sorted out and would propose a visit to a neurologist ASAP. Hope this approach is of value.