Three week old chick slight bubble sound with respiration

[Godiva]

I have noticed that one of the BO chicks is way smaller than the others, she seems to have stopped growing. She is in the brooder with a bunch of other chicks and everyone else seems absolutely fine. She tends to just stand around all fluffed up. I haven't been able to catch her pooping so can't tell you what that looks like but her butt is clear of droppings. When I put cut up greens in for them all she does show interest for a little while but doesn't seem to have any energy. When I picked her up today there was a lull in the general hubbub with the others and I could hear a sound kind of like a tiny bubble popping every time she breathed.... What should I do? I am guessing I should isolate her incase it is something infectious? I am wondering if she has got sick because she is not strong or if she just has been sick longer than I realised and therefore is not strong... I am not sure what other info you might need. She doesn't seem to be feeling cold as she is not sitting under the heat lamp but is fluffed up all the time...

I can't afford to take her to the vet... and I am not sure about where on earth I am going to try and isolate her... I guess I am also wondering if it would be better to put her down quietly tonight

You know how fast they grow at this stage and she is suddenly obviously way behind the others on weight, probably about a third less than the other pullets! Do you think she might have picked up pneumonia? They are all on unmedicated chick start n grow with kelp meal and diatomaceous earth added. As I said the rest are thriving...

 

[Godiva]

Well, DH made a plan so our little sickly one is in a plastic tub next to the brooder so she can hear all the noises but is able to rest. While I watching her I could see her panting slightly and can still hear the bubbly sound with each breath. I was giving the others a few greens and cut a few little bits up for her and she has been eating them... but she is really listless and quiet. I am hoping that having some peace and rest will help her fight off whatever this is. Also now that she is on her own I can see if she is pooping and what it looks like. What else should I do?

 

[Godiva]

Is anybody out there?! I checked little ones poop this morning and it looks fine and normal amount around too. She seemed pretty keen when I dropped more greens in. She doesn't lie down that I can see, just stands and pants and makes that tiny popping bubble sound with every breath. Should I give her antibiotics? Where would I get some? What do you think she has? Is this something infectious?.... HELP!? I have never had a sick chick before so this is totally new for me....