Sluggish and sleepy 26+ week Buff Orpington hen

I have had a hen for 21 weeks (she was at least 5 weeks old when we got her). We also got 3 other hens with her. The other hens all started laying over 4 weeks ago, while this hen never has. I wasn't concerned, but started noticing a few days ago she was very slow walking and preferred to be by herself. Then one morning stayed in her coop sleeping while the other girls were let out to free range. When she did go out she did not scratch like the other hens, but would peck for bugs. I checked her feet and did not see anything wrong. Her breathing was a little labored when I picked her up (but she doesn't liked to be held so could have been stressed). She has no nasal or eye discharge or discoloration to comb/wattles, or lesions I can see. She has not lost any weight. I separated her from the flock and put her in our sun porch with food and water. She does eat and drink a little. Her droppings look normal, although have lessened. She just appears to be sleepy and sluggish. Any ideas? My other hens are fine. They get out daily for several hours to free range. They are given organic feed. It has been really rainy the last few days and wet in their run - although their coop was dry and I put new fresh straw and bedding in daily. Any suggestions would be helpful.