Could be. The flat cold/warped hot phenomenon is sometimes observed, but usually on older, thinner pans. Are you sure it's a cold/hot thing? The bottoms can be curved but the rings still flat, so they'll be flat on a counter, but can wobble on a grate, depending on the size of the grate and the pan. Are they sitting on the rings on your burners, or do the rings land outside the grate? Or the grate might not be perfectly level.