Hi , I Have Braces And I Have Elastics On

Hi , I have braces and I have elastics on my teeth in the front to pull down the two teeth on either side of my canine and now I have a mouth ulcer. I have already put salt on it and it now has a black dot in the middle of it as well as a lot of swelling , I don't know why it's doing this and I frequently get ulcers but this has never happened before. Please Help.