Problems wit ex gf?

ok...about 8 months ago i broke up wit this girl, she neva called after that and in school she neva talked to me. now she is acting differently around me, she is always flirting, and then she asked me back out today...but the thing is when i said no to her...she gave me a ton of grief about it...i jus dont care for her anymore and she dont understand that...what do i do?