Sounds like your tank is cycling - what's your ammonia reading? Ammonia is extremely toxic, likely the root cause of the angel's condition.
Has the angel's behaviour changed at all? Is s/he eating, swimming normally, breathing hard/fast?
What's the texture of the red area? Raised, smooth, raw looking? Does it have distinctive edges, is it the same on both sides of the fish? Get as descriptive as you possibly can.
Water changes never hurt. I'd start changing 50% daily - even if you don't know what's wrong, it will at least keep NH3 down and reduce stress on the fish.