Sharp pain low left back for a month+. in pt for post l4-s1 fusion. therapist said today he doesn't think its muscle maybe hardware. if so, now what?

Many possibilities: How far post-op are you? How are you progressing in PT? Could be anything from benign muscle spasm of increased activity post-op to infection (unlikely but...). Could be you are feeling the hardware if you are thin, but not usually a problem. Body takes time to adjust post-op, so if infection not an issue wait it out and take it slow. If concerned, discuss w/ your surgeon.