Maybe she ate a cat and it's stuck between her teeth. Some cat floss might help.
OK, on a more serious note, there may be something stuck between her teeth...like string, hair, a piece of a toy or bone...or she may just need a dental.
Our Zoey went to the Vet with wicked bad breath. We were going to have a dental done...teeth cleaning. Vet looked her over and said her teeth were great. The smell was coming from under her rope. We had not been so diligent with our "fold cleaning" and she had developed an infection. Some topical antibiotics and regular cleanings for one week and she was smelling delightful again. We are are better with keeping her folds clean now.
Nyala doesn't appear to have much of a rope but this is just an example of what might be going on. Sometimes what's obvious to us ends up not being the problem.