Well, I think the first thing would be a vet check - you are right and she could be in pain, especially if this started suddenly. Egg binding is one possiblity, very painful and needs to be treated immediately. But there are other possibilities too. Some cockatiels can have night frights and flap in their cages and break or dislocate bones... or maybe she could have a crop impaction that prevents her from being able to eat much and she's crying from hunger. I'm no expert, just thinking of possiblities.
If she's acting normal otherwise, eating well, looking relaxed and playful, and not favoring a limb... then just watch her closely. But a vet visit would not hurt.
She might see something new outside s window that scares her (hawks, crows, a flapping flag or tarp, ect.). You can try placing her farther from a window and placing something between her and the window like a plant - so she can see out a little but feel more secure.
Or she might have learned to scream for attention. If you always go over to her to comfort her or quiet her when she screams... then she may have just figured out screaming gets her attention. But, if the screaming just started suddenly one day, then this is unlikely.


Karen
Chico (conure) and Pippin (budgie)
WI