virgin mobile overcharging on their everything plan w/o reason, Internet

Since 12/28/11 Ive been disputing a data charge on my phone with virgin mobile.first it was a dollar ,it was settled.then again on 1/8/12 it was a quarter. my question is if it is a 55 dollar unlimited everything plan why are they taking any money from my account.i am disabled and only get paid once a month. i explain this to them on the 28th of december and all they coukld say is that their backup office  was looking into it. i've been with virgin mobile more than 5 years so what is the problem now,i've received no response from them since their last e-mail on the 8th of january.what can i do?