I had an L5-S1 TLIF with instrumentation 2/12/15. Since, I have had a rectangular swollen area about 1.5" above the fusion site. Any ideas?

Seroma. Multiple potential causes, I would discuss directly with your surgeon. One cause that is fairly common is to have developed a "Seroma" from your surgery. A Seroma is a fluid collection that can develop in healing tissues and often slowly resorb a over weeks to months after your surgery. Again, I would just make sure your surgeon is aware.