It sounds like a fur slip, especially since it likely happened during playtime. Chins will slip fur as a defensive mechanism. When a 'predator' (or human) tries to catch a chin, their fur will release leaving the 'predator' with a hand/mouth full of fur and no chin. This will grow back and quicker than you might think. Usually you'll see new fur starting to come in within days.
If you see patches of shorter fur (not bald patches), she may be chewing. If she has bald patches not from a fur slip, then she may have fungus which you should start treating right away. The sooner you catch it, the better.