YES, mouth sores can be a sign that you are having an allergic reaction to Nickel, but there are other symptoms (which I have).......
Skin reactions such as itching, burning, redness or other rashes are common symptoms with Nickel sensitivity, and asthma attacks are another, but less frequently seen in people.

I personally tend to get the over all body rash that comes and goes when I come into contact with Nickel, in food and items.... plus I suffer more from sinus problems when to much Nickel gets into my body, via vitamins and food.

SIGH!!! What can I say, but that I have become extremely sensitive to Nickel this past year.... one of those rare few people that fall into the 2-5 % range that will develop Nickel Allergies.

Thanks for the advice with the suggested experiment - I have done something similar to that and it helped some, but not enough.