a nick.
Her surname could be Nitkowska, then Dornitka would be a nifty nickname
Also, nitka is a thread. Is she very slim? :)
Surnames ending in -tka are very hard to come by in Poland, I guess.
It's probably her nickname... almost positive, 'cuz it isn't a dimunitive of Dorota (which would be Dorotka) if you say "Dorotka" it is a nicer way of saying it, it makes it sound more 'cute'. if you say "Dornitka" - sounds even cuter although I wouldnt call it a dimunitive - for me it's something more like neologism... :) calling a girl called Dorota -Dorotka, Dorocia or Dornitka Is like calling a guy called George - Georgie... This is how I see it.