Staggering Chick

[Yumachick]

We are new to chickens. Have had 2 bantams since July who are just fine and housed outside. Got 3 day-old barred rocks on October 18th. One of the babies was a little unsteady starting a week or so ago. Kind of like she had a leg that fell asleep. This morning she was very wobbly, staggering. Seems like she doesn't know where her legs are. When she stumbles, she tends to put a leg backward rather than to the side. We have separated her from the other two and have been searching the internet for answers! This morning I splinted her thinking spraddle leg, but she seems a bit old for that. Since we got them, the have been kept in a clean crate in the garage with a heat lamp - no exposure to other chicks, soil. The crate is lined with folded towels that are changed at least once a day. They started on chick mash and were switched to chick starter about a week ago. Always have clean water. Sporadically did probiotics and electrolytes in water for first two weeks. Plain water now. She is eating, drinking, peeping and grooming fine. Poop is mostly regular. She had a sticky poo earlier today (consistency of peanut butter - didn't see her do it, it was in her crate). Any ideas?!