5 years ago, I had 2 rods and 4 screws put in my back at l-5, s-1. If I am now having pain and stiffness in that area and all the way across my back, from hip bone to hip bone, what could be causing it?

See your surgeon. You really should discuss this with your surgeon since he/she was there and can give you all og the what ifs.
Maybe Sacroiliitis. You have to see your spine surgeon. He/she will need to get x-ray to ensure your implants are in good position, and there are no breakdown above your fusion. You will also need to be examined to rule out bilateral sacroiliitis, which can happen with fusion to the sacrum, which in turn increases stress about the sacroiliac joint(s). Treatment depends on the exact cause of these symptoms.