Is she doing this continuously or just every now and then when she's basking. If she's doing it every now and then just keep an eye on her. If she's got excess mucus in her mouth then she could be showing signs of a respiratory infection.
Symptoms to watch out for:
The presence of increased mucus in the chameleon’s mouth.
Weird noise accompanied with breathing (popping, crackling, purring, or cat-like sounds). Try to keep your ear closed to your chameleon and listen carefully for these sounds.
Labored breathing by gasping and opening its mouth even in the low temperature range. In the morning, chameleon does yawn like human. Such activity is normal.
Lungs often overly inflated.
Some swellings the forehead (between eyes) and even eyes (looked puffy) can occur.
Snoring during sleeping.
This information is taken from here
http://chamworld.blogspot.co.uk/2008/01/upper-respiratory-infection-warning.html
If she is showing signs of an infection, then get her to a specialist exotic vet.