circular patterns on background?

I photographed a family on a white BG but I didn't light the Bg. I added light to the bg with a 10 degree grid to give it seperation. I con verted the photo to B&W and used the NIm Software and used the lighten center darken edges tool. I made a print and there are circular patterns on the background, I even did a gaussian blur on the BG but it still is there. Any idea what it is? It kinds looks like moire