It's really hard to say. It's common to see this when there is some kind of anxiety or pain, though.

The smell of a dogs mouth can be pretty stinky. It's usually because we don't brush them daily and they lick and chew on everything and bacteria just smells bad.

If you raise her lips and look, the teeth should be white everywhere and the gum line should look healthy like ours.
If there are brown spots, thick tarter, pockets of gums that droop or even pus, then she has serious teeth decay and a dental cleaning woul help not only with smell but in the heatlh department as well. One of the leading causes of heart disease, pneumonia, and kidney/bladder infections is the bacteria from the mouths of these rotten teeth dogs.

Aside from this, things that they eat can cause the smell. Little dogs often steal cat poop treats and this can cause stinky breath.

If she licks around her bottom, she could be licking her anal glands (produce a foul substance when the dog poops) and then the secretions from the anal glands get on the mouth and this really smells bad. If you suspect this, your vet or a groomer can express the glands and help with this.

Lastly, when there are internal changes like kidney disease and diabetes, that changes the smell of their breath.

This would be a good time for a thorough exam by your vet to rule out some basic problems (parasites, etc).

I hope this helps!