Hi there, hope you are enjoying BYC!
Is this a pullet we are talking about? If so, it is very common for new layers to have hiccups while their reproductive system works out all it's kinks. Sometimes even coming back online from molt can throw weird eggs. They can be effected by lighting changes as well, maybe causing the early release of another ova. This is true for slab sided eggs as well. Have you already seen this article...
https://www.backyardchickens.com/articles/common-egg-quality-problems.65923/
A few liquid poops is quite the norm since all urates are passed in that manner... but it is usually still much different than diarrhea. Also, sometimes a sneeze is just a sneeze... it always important to take not of ALL the symptoms present so we aren't in a stressed out panic every time something minor present.
Being described as underweight... there isn't a whole lot of consistency among hatchery birds and they can vary quite a bit among the same breed, even from the same place and same hatch... in my experience. If she is eating and drinking, and not being bullied even at the bottom of the pecking order... she may be just slightly smaller in general.
What's interesting... I have NEVER had a Rock at the bottom of the pecking order. They usually head the flock.
Two things that might have some impact... nutrition... too many treats diminishes nutrition, (usually decreasing protein or amino acids and increase fat or carbs). I personally never use "layer" anymore since learning the difference, 20% protein flock raiser with oyster shell on the side for active layers... and you can't go wrong, IMO. Getting a group fecal float can tell if their may be internal parasites since only large round worms or tape worms will may ever be seen in droppings and all others stay int the intestines (taking valuable space and resources) and only their microscopic oocysts (eggs) will pass. I personally NEVER treat without a float... as so many things people SWEAR are worm symptoms have came back negative from the vet. Do a float a few times per year to see where YOUR parasite load sits. Some folks environments DO warrant treating on schedule, not because they're bad keepers but due to environmental factors like weather, soil drainage, wildlife load, etc. There are some mail in labs that may be more affordable than local vets. This is a much better choice than spending time & $ treating what you don't have while maybe NOT treating what you do. So consider looking into this and maybe set your mind at ease?
Now for me, winter is heavy external parasite season... lice, northern fowl mite etc. One year I dealt with them and since then, check the birds once a month AFTER dark while on roost, keeping lights dim and using a flashlight or headlamp... as this is when it's easiest to not chase anyone but also to SEE the things running away from your light. It is FAR more revealing than daytime inspection. Also some things like the Red Mites live in cracks of the wood and only come out to feed on the bird at night, NOT inhabiting them, just feeding on them. Running a white paper towel under your roost a few hours after dark can reveal if your coop is harboring them by the little red blood streaks that will appear on the paper towel. Again, just like the internal parasites... don't treat unless you verify need, in order to prevent resistance by the target parasitic species from building.
Hope this is helpful without being overwhelming.
Lets see her condition, pics always welcome!
