Severe pain in lower back, leg & buttocks. can't sit down or walk up stairs. pain even too intense to walk. could this be sciatica or worse?

Sciatica: This could represent sciatica or purifier is syndrome and warrants further evaluation

Yes: Yes, "sciatica" describes the irritation of a nerve in the back that then causes pain that runs down the buttocks into the leg. If it does not improve, or if it is associated with numbness, weakness, or bowel/ bladder problems I would be seen. On the other hand, many cases can resolve on their own as well.