3 month old female Indian Runner acting Strangely

[FlyOverZone]

I have about a dozen ducks. 6 of them are runners. The oldest one is a White 3 month old female who in the last day or two began acting "quiet". I let the birds out to free range, but she now prefers to stay in the pen. she keeps her bill tucked close to her heaved chest. she walks around with ZERO energy and sleeps A LOT. I have seen her get up to drink water. She doesn't seem to be limping or wounded. She's just QUIET and sleepy looking.

Any ideas? Anything I should check or give her as a preventative of something?