recent siege in which some dwarfs perished. She has been very unhappy lately.
... many thoughts...
One thing to check is her relationships (which will also list her pets). Does she have relationships to recently killed dwarves? This -should- leave a message in her thoughts, but maybe didn't for whatever reason, or fell off the list from other thoughts? Would be weird, but would be a way to double check.
Ways to calm her down - Isolation and time are perhaps wise. Her pets can be pastured away from her (though that risks those who move the critters away) and/or she can be drafted (more bad thoughts in that drafting unless she is already military trained) into her own squad, then ordered with s to move somewhere, which you can then seal.
This is unlikely to calm her, and you have other upset dwarves too?
"therapy time" can be calming, especially if your dwarves are already mostly friends with each other (Making friends is very pleasing, but once made deaths and such cause great unhappiness - if you already let the friendships start why not use them now?) Don't set any unneccessary new jobs, and turn off whatever labors you can. Make sure you have just one meeting area, and that a single tile. Most of your dwarves should cluster there, with "no job", and socialize. This is very cheery, and groups them close together for mutual defense if that dwarf you want to save does go berserk proper before she calms down.
As my playstyle does permit dwarven friendships to flourish, this method is the fastest means I know to raising the mood in an otherwise "everything is ok, now get over that just-finished tragedy", and usually works well.
If you do not want to allow more friendships, you'll have to find another way to help her