This is very common with pet cockatiels. As for the egg, she will most likely lay several more, every other day. If she doesn’t have a cuttle bone, cook her an egg with the shell washed, crushed and cooked with it. This will give her some calcium and protein to help replace what she loses with each egg. This should not be fed to her except for right now while she is laying eggs. You can leave the eggs with her, but don’t give her a nest or bowl or anything. She can sit on the cage floor, even if there is a grate. Hopefully she will have no interest in the eggs, If so, just throw them away. There are several things you can do to discourage egg laying from this point.
You need to do all of these things to discourage egg laying. Keep in mind that to lay eggs, she needs longer daylight, warmer weather, abundant food, and a quiet, private environment. Your goal is to reverse these conditions.
Limit her light to 8-10 hours by covering the cage early each evening
Do not give her anything to use as a nest – no bird huts or tents, no box, bowl, etc. If she decides to sit in a food bowl, remove it and replace with smaller cups.
Do not give her anything to shred such as paper or cardboard.
Rearrange the toys in the cage frequently.
Move the cage to a different place in the room. Move the cage about once a week, or whenever she shows signs of nesting – settling on the cage floor for example.
If she is let out of the cage, do not let her get in any dark cozy places and don’t give her free roam.
When you handle her, limit any petting to her head and neck – do not pet her on the body.
If there is no metal floor grate, then do not use any bedding or paper in the cage tray – leave it bare and clean it daily.
Happy New Year!