7 month old hen underweight and lethargic. Not sure what to do?

[Readstochickens]

I have a ‘runt’ hen, she’s 7 1/2 months old. She’s always been the smallest, she’s also the youngest by 6 weeks. She’s, what I was told, a Columbian Plymouth Rock. She’s smaller but more shockingly, she’s much lighter than the other 7 hens (2-14 months old (GLW), 5-10 month old (3 GLW, 2 Blue Plymouth Rock) Yesterday and today when I come out to give treats, clean the yard, run and coop, I noticed that she was alone and sleeping. I’ve never seen her sleep with her little head tucked in her wing besides at night in the coop so it was very odd seeing her doing that in the yard. Yesterday I noticed her bum looked a little wet as well. I always try to isolate her when I bring treats to make sure she’s getting them before I go back in. I don’t know what wrong with her and I’m really worried now. I’m thinking coccidiosis or worms? I find shell-less eggs about 3-4x’s a week sometimes despite calcium being placed in the coop, run and yard. What is your opinion? What treatment do you think I should try? I’m very worried about her, I love her. Thank you all!