I've never been good at identifying fish diseases. It always seems kind of hopeless to me. I'd advise more water changes. I found two links to lists of general fish diseases that may be useful. You've probably already done a similar search and found similar material. The links below suggest that it may be bacterial.
I couldn't find anything about a swollen mouth but did find many references to cloudy eyes. In most instances of cloudy eyes, the attributed cause is poor water quality.
The link below advises to check the pH of your water.
Maybe try a salt treatment.