I have a hen, Leeya, who has not laid an egg for about 3 weeks now. She doesn't appear to be broody as she doesn't sit in her nest or go spacy. She is up and around, but she doesn't appear to be her usual spry self. She's a bit slower than usual and does some odd things like leaves one foot kinda dragging behind her or will put one wing down on the ground. Sometimes her tail will bend to the left and she'll walk around like that for a while. Granted none of this lasts very long. But was does last consistently is the shaking her head - she's doing that a lot as if to shake off flies. She will also hang around the water tank sort of leaning up against it or squatting next to it. It's as if she's in a daze but she's not sitting in her nest. She looks a little "unkept" too but not overly so. Also, when the other chickens free range she used to get right out there with them. Now she just stays in the pen area. Details: I have no idea what type of bird she is and neither did the man who sold her to me. He said she was a mix of many different things. She did lay greeninsh-blue eggs for a while and then just stopped early May. When we got her in October of last year she was very young and had just come from the needing heat phase (I'm sorry I don't know what phase of life that is for a chick). The below photo was taken early this year before her odd behavior started. The second photo was taken today.
Is there a certain kind of illness that I should look for? I don't understand what is going on with this chicken. She's got me stumped. Since I'm pretty new to this I really have no idea what I'm looking for. TIA!