It is hard to know if bacterial or viral, but if this is your only bird, and there is no discharge, and it is not likely you've got gape worm (look down her throat with a bright flashlight to see red worms in the esophagus)...then in my experience it is likely MG.
I've used Duramycin-10 in the water for 7-10 days, following the directions on the package...I think it is 1/4 tsp per quart, 1 tsp per gallon...but check if you go that route.
Many prefer the Tylan, but if you prefer not to use needles and inject (not all owners are comfortable doing that), I've had good luck with the Duramycin-10 for MG type symptoms (Chronic cough/rattles).
The other route is assume it is viral...but usually viral goes quickly through the whole flock. Bacterial seems more selective and slower.
If you know (or highly suspicion) viral (I had a case of IB go through the flock one year), the initial stages were helped a lot by placing elderberry tincture in the water and adding fresh garlic mash to the feed. You may then consider antibiotics to limit secondary opportunistic infection that grows with the extra mucus from the viral. But usually with viral you get some watery eyes/nose discharge and more than one bird at the bare minimum affected.
I would isolate the bird until you figure out what may be going on.