Apparently, this one has been around for a while on Apple’s support forum and is so rare that it isn’t considered a problem. Anyways, randomly an iPhone can increment its cellular data usage counters up 4GB both up and down simultaneously. So you could go from say, 15MB down to 4GB up AND down.
This can have a rather un-desirable un-healthy reaction. Simply put, if your iPhone does the jump and you catch it and you haven’t been tethering or doing unsavoury things with it, relax. Your carrier doesn’t rely on your phone for your bill, so you’re safe!