Chicken walking like a duck-should I be worried?

She seems healthy and behaves normally (eats, runs around with the other chickens) but sometimes she waddles around like she's a duck with her knees (?) bent and her belly close to the ground. She does this for a little while daily but then resumes walking normally. When she tries to run in this position she stumbles around. She's been doing this for about a week and a half on her own. Why? None of my other chickens do this. Is it a sickness?
I have 2 pekin ducks, maybe she is pretending to be one also?

Today I petted her on my lap. She stood up and flew off, walked, and then crouched down and proceeded to waddle again. And right now she is walking normally again. What.

Will provide pics if needed.