The only way on PAYG is a "subject access request" which costs £10 so is impractical in this case.
http://www.o2.co.uk/help/safety-and-security/how-to-get-a-copy-of-the-information-we-store-about-you
Most likely explanation though is mobile data being on and using credit for background tasks.
Turn off mobile data and just use wifi for internet, emails etc and it will preserve your credit.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
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