well, she is a mother. She is concerned for her daughter. Yet she is hurt that someone has hurt her daughter and put her in the position of an unwed mother.
A girl comes home pregnant, then produces a certificate, stating she is a virgin and then immediately, the girl leaves home with her bags and stays out the whole night, a Mom is bound to be worried.
Anu's Moms past is not yet out, but she is a single mother and a weak woman at that, it appears. She confesses that she cant be as strong as Anu. So, she is bound to be both irrational, anxious, worried and hurt by her daughters action. She is still living in a Society where unwed mothers are frowned upon, especially when the mother in question is a teenager. Poeple are bound to talk, raising questions on her integreity and honour. Which mother will be able to calmly face this.
Some Mothers can. But the character description of Meera is such that she cant accept it and she cant take it calmly.
Probably, one of the reasons for Anu to be so confident is the weak attitude of her Mom as well. Anu knows she has to be strong, her mom cant offer her emotional strength or financial for that matter. The inner resolve of Anu, coupled with Meera's attitude, altogether makes Anu what she is today and how she copes with teh situation.