Why did Mona's name change in the later part of 3 idiots?

Mona Singh, whose name is Poonam, becomes Mona in the later part of the film 3 Idiots. After squabbling with Viru Sahasrbuddhe, Rancho suddenly starts calling 'Mona Push'. AFter that everybody, including her father, starts calling her the same name.

Is this a movie mistake or was Mona her nickname in the film?