Greetings
JuliaWesterbeek,
She appears very ill, sleepiness during the day is not good.
More information is needed. Here are a couple of common health issues in young chickens, to check for.
Crop:
Have you checked the pullet's crop? Her crop should be empty in the morning. Feel her crop right now, is it hard with food? If she isn't eating much, there shouldn't be much food in it. If the crop feels puffy and doughy, and has fluid in it, next, smell her mouth. Does it have a stinky odor like something fermenting, or something foul?
Poops:
What do her poops look like? Diarrhea, firm, lots of clear water? What color are the urates (the white part)?
A photo of her poop would be helpful.
Check the hen's vent also, to make sure there is no sign of irritation, blood, protrusions or swelling.
Have you done a fecal float test on the hen's poop?
Coccidia:
Coccidia are found all over the world. Because of the pullet's age, she may have coccidia. She has many of the symptoms of coccidia. A fecal float test would verify this. However, you can start treatment right away, it won't hurt her. Corid (amprolium), is a common treatment here in the U.S. It is mixed in the water. But, if she isn't drinking, you may have to administer some orally. Another treatment is Albon (sulfamethioxine), it can be found in a liquid suspension and administered orally. Albon also treats other infections.
Do a basic exam:
Check the hen's body, under the wings, under the feet, etc., for injury, lumps, growths, and external parasites. Many health issues are made worse by secondary problems.
If you have access to a vet, that would be the very best thing for the hen. If not, you will have to do an exam on the hen and compile a list of as many visible symptoms as you can.
The members here on BYC will do their best to help determine what is causing you pullet's illness.
These are my thoughts on your chicken's issue. I hope I have been helpful.
God Bless