Lower back pain with weakness in legs. what could be causing this?

Lots of things. You need to write down your symptoms & timeline and take it in for an in person evaluation. A review of history, exam and testing are how this gets sorted out.

The lower back pain could signify lumbar stenosis which could cause neurogenic claudication but you might have vascular claudication, or even a co-morbidity such as peripheral neuropathy. Since these are merely random guesses, you truly will require a careful and thorough examination with appropriate testing to achieve clarification. A neurologist may be useful.