She may have been pecked in the eye or got some debris in - let's hope!  I would monitor her eyes for a few days.  Generally with respiratory illness the bubbles and symptoms continue.
I'm not sure what type of troubles you have been having.
What do you feed including treats?
I'm not sure what you can do for immune boosters.  
@coach723 may have some suggestions.
I don't normally use any medications or supplements for the whole flock - I do use them 
sparingly for an individual that may be feeling poorly.  Fresh feed, fresh plain water daily and a small treat most days is mainly what I do.
		
 
Fingers crossed--I think that may have been what it was. She's been upbeat all day and acting fine. No more bubbles and eyes look bright/clear. 
My troubles have been a cascade beginning in the summer. First, a couple months ago, my most beloved EE hen had a soft-shell egg break inside her, and we've been going through a long battle with EYP. She seems 100% better, and now I'm working on getting her transitioned back out with the others--but she is in a SERIOUS molt and very frightened of the other chickens. She prefers to hang out on my arm and will jump on me when she wants me to take her back inside. 
Meanwhile we had an initial battle with suspected worms (all the girls were looking droopy mid-summer), then a long, drawn out battle with mites/lice that we finally won, and some issues with molting hens acting "off" in a vague way (droopier, eating less, a few soft-shell or broken egg incidents). Everyone seems okay now, though I'd been monitoring them due to some intermittent weird poops--watery, intestinal lining, whole strands of undigested grass--and that's when I found the roundworms. I'm now cleaning their runs of all poop every day to prevent the worm issue from taking off any further. Happy to report today's batch of poops have all looked normal. 
The girls were recently switched to Purina flock raiser due to their molt and have been doing well on it. Oyster shell and grit is always out, free choice. I give them mealworms as a treat to lure them back into the coop during supervised free-ranging. Once in a while I make them a treat like a batch of soft scrambled eggs, some mash with crushed garlic or probiotics, or today, for example, a hunk of fresh pumpkin with chopped up seeds. Their diet is mainly feed, though. ACV in their water intermittently. 
I think my anxiety has something to do with it. I tend to read too much into things and panic/react quickly. That's because: after my first 1.5 years of having chickens, I've learned--the hard way--that things can happen subtly, yet quickly (and it can become too late in a blink). I know that half our flock wouldn't be here had I not been watching them like a hawk and being proactive--which only fuels my constant worrying. And, in their first 1.5 years of life they have been through the gamut: heat stroke, mild-to-severe bumble foot, c. perfringens, lice/mites, worms, EYP & related crop issues, cracked beak, etc. They are troopers, and I have managed to pull them through these things, sometimes by the skin of my teeth. I always feel guilty they have so many issues, though! What am I doing wrong?