To find out the reason you need to do thorough investigation

She required admission may be iron levels are also reduced.. requires intravenous iron injection in 100 ml normal saline for 4 to 6 weeks.. and require some investigation Also

She need to start with the treatment..

Are her periods very heavy?

Anemia with Hb 7.1 is significant. Need to evaluate further to know type of anemia. Most common type is iron deficiency but nit common with Hb 7.1 on presentation. Other possibilities are anemia due to any chronic disease or blood loss in period or hemolytic anemia. Need detailed history and evaluation to start treatment.

There r so many causes of anaemia

Needs history. Detail evaluation for causes of anaemia. And acordingly hematinics and b12 for treatment