Leg paralysis is a classic sign of Marek's. If she has no visible injury or outward sign of illness beyond the the leg, and is eating and drinking- you need to consider this. Look it up, lots of threads on it. If you have a specific question on Marek's after reading about it I can try to answer it (vaccines, supportive care, flock exposure ect). It is a hot topic, as it is in most people's backyard flocks- very sad and frustrating disease.
Trauma is another possibility- leg stuck in something or grabbed by a dog ect. If you feel nothing broken, and cannot take her to a vet for x-rays of the pelvis and the femur (which is not very 'feelable' in a chicken from the outside)- then all you can really do is supportive care.
Botulism's other name is 'limber neck' if she has her head up and is perky and eating/pooping- it is unlikely she has a systemic toxin causing paralysis.
Do you know for sure she is not broody? If you take her out of the nest box and put her on the ground outside- what does she do? Are both legs splayed out, or does she stand on one and hold one up? Drag one behind her? Can she hold her wings up, or are they down?