Invisible fire in the nether

Every time I go bed bombing, I am constantly hitting invisible fire. I can't see it but I can hear it and I take damage - but don't catch on fire - when I walk through it. It only appears after igniting a bed in that area; I extinguish the other fires but the invisible ones remain and can't be extingished. I have placed blocks on the squares the fire was in and it doesn't put it out, it just comes right back when I break the block.