Soot on any part of a fire-type’s body is an indicator that its body temperature is dropping. That, in turn, is typically caused by exhaustion, stress, or a mild cold. My advice would be to let your fennekin rest and perhaps take her to the pokémon center if she seems lethargic, her nose feels dry, or she’s having trouble eating.
Best of luck, anonymous!
The Pokédex According to Bill 