Is it only skin that's in contact with metal, or is it the band? Has there been any wear and tear on the parts of the watch that seem to be causing the rash? If some protective coating has worn off, it's possible that your skin is reacting to the material underneath.
Does this only happen when your wrist gets sweaty? The moisture and/or heat could lead to rash. It's also possible that your skin has gotten sensitized, in which case there may not be much you can do to avoid it.