Subdued chicken after missing one night

Here is what is going on with one of our girls. We let our girls free range and they always return to their coop at dusk when we lock them up. Two nights ago Lady Gaga didn't come back- we searched everywhere and couldn't find her. We have a large property including a huge barn that they love to go under so there are lots of hiding spots. There were no signs of a struggle and as of yet we haven't had any issues with predators, but I did of course consider that that may have happened. The strange thing is that Lady Gaga is the funniest of our 5 Sex-links. She is often away from the other 4, just doing her own thing but she follows me around- will come into the house, barn, wherever she can find us and seems to be the one that doesn't wander away very far. It was really odd for her not to return to the coop that night.
Anyway- the next morning, around 7am there she was outside the coop. I let her in and she went straight into the nesting box. I'm not sure whether she laid an egg or not.

Now since then she has been very subdued- sitting in a corner outside, then sitting in a corner of the coop by herself. Last I think she roosted on a low branch by herself.

I found a rubber egg on the lawn yesterday afternoon too- not sure if it was hers.... Could it be related to her behavior?
She seems to be eating and drinking a little, and she does walk around some too but she is definitely not herself. Lethargic, subdued, by herself. Normally she runs to the coop door when it opens but she has been staying sitting where she is.... She also looks to be holding her tail up and ruffled some.

So- anyone have any ideas? Stress? Bound egg? Sick? I just want to try and figure it out before she gets any worse!
Thanks!

What do you mean by rubber egg? squishy egg? Just trying to clarify.

She could've ate something she shouldn't have while she was out. I used to have a golden crested polish hen (Tina Turner) that was towards the bottom of the pecking order. She was notorious for going on "vacation". She was trying to avoid getting beat up on the roost. She would show up a few days later. The neighbors would all feed her, and she would just hang out there a few days, sleeping in trees and on top of their roofs. Maybe it has something to do with the fact that you named yours after a singer as well (Just kidding ).

I would carefully observe her for a few days. If the egg is squishy, give her oyster shell. You should have that readily available for calcium for the layer hens.
When she poops, check out the poop and make sure it doesn't indicate an illness. There is a poop chart, let me know if you don't know where to find it.
Are her feathers fluffed out? The tail up, is generally not a bad sign. Tail down is.

By the way, Welcome to BYC!!

Thanks! I got home a little while ago and she was out and about so I'm hoping it was just stress..,. And I think she's the lowest in the pecking order so that probably doesn't help!
I will keep an eye on her though and let you know if anything changes.....

By rubber egg I meant the shell was very soft. I read that can happen with stress sometimes?!? And yes we do have oyster shell available

(My kids names the girls.... We have a flock of divas lol)

Sounds good. Keep me posted

Is she a new layer? Soft shells are pretty common in the first month or so, and they can act rather weird when they lay them.
She might have had a scare while she was out that night.