As elmister said, check your stats if anything strange happened, and even run a rootkit checker. You should ask your webhoster to help you.
50 Gb's in one day could even be used for other things... some traffic can't be seen in control panels.
Keep in mind that the bandwidth might not be used by Apache, but by FTP, mail, some other daemon or as someone already said.. an exploit.
Are you on a managed VPS?
- If so, and your host can't find the cause - they should either be able to give you an explanation why they can't find it (it might make sense if they say there are no logs indicating such usage, and no traces of an exploit). If they can't get you a proper response then it's probably best to move on elsewhere.
- If not, then I'd suggest moving to a managed VPS account.
the code was injected, not sure what they did, the bandwidth shot up, we saw it on our router.. and shut it down quick.
We did a rootkit check afterwards, not found anything.. but we have a fair idea where it comes from.. there is a phpNuke that is not patched up to date.. so we get the customer to patch it up, then give them a new VPS to move over and destroy the old one. You can never be sure whether they are gone for good.. but just do as much as you can and monitor.