why do I need friends... [help me]

when I have enemies like her?

so this girl was my best friend for a while.
then she turned into a total bitch, getting mad at me for no reason.
so we stop talking, I get my shit back from her, CDs mainly.
she forgets to ask for her calculator back [one of those expensive graphing calculators]
now she needs it for her geo class.
but she still owes me two CDs that got lost while she had them, and $20 for a concert in AUGUST.
is it wrong to hang onto her calculator until I get my stuff back?

what sucks is, my current best friend, at least I thought so, is siding with the old one, and I don't want to lose her just because I'm trying to get my stuff back.

this is a big ole SOS.
help meeee.