I totally agree with everything in that excellent post , Millie! How many times have I groundlessly diagnosed horrible things for myself and my animals with the help of the Internet. But at least nowadays one can get a lot of information about what's really important to look out for, and also find out what the basics of good husbandry based on common sense might be. When I got my first chickens in the 1970s, I really had to play it all by ear and made lots of unnecessary mistakes along the way, as even books on poultry keeping for beginners were practically non existent and very expensive as well.
Yes you're right about not seeing the worms, except in very severe infestations, but it's still a good idea to keep up the worming, as one would for dogs and cats, because then you can protect the flock against more serious infestations which would pull them down and make them less well able to resist any other health problems they may encounter. Also, worms take a lot of nourishment which should have gone to the hens themselves, to help them maintain strong and healthy bodies and produce lots of eggs. If you worm regularly. At least you can rule that out as a factor if one of them is under the weather. I don't suppose worms caused her problems as she's a young bird, going through stress at an unfortunate time in her development, but all the same she was due for a treatment.
By the way, don't feed them earthworms, although they love them and they provide a nice protein snack, because earthworms eat contaminated soil if your birds have been out on the grass and are an intermediate host for worm eggs, which then get back in the chicken from whence they came. Not a problem for you at the moment, more of an issue for people where hens have been on the same grass for quite a while.