rust stain in bathtub

Hello, this is kind of an odd question. We recently had someone come clean our home. Now, in the bathtub, which is old porcelain, there is sort of a rust-colored gully running from the drain, merging into kind of a lighter spatter pattern under the faucet. It won't come up, and it appears that the enamel of the tub has been eroded in that area.
My girlfriend believes that it was never there before. I think it's been there there all along, but it's now in sharp relief with all the rest of the muck (and there was a lot of muck) around it removed.
It turns out she cleaned the tub with liquid bleach, which we don't normally do. The question is, could the stain have been caused by cleaning the tub with bleach (meaning it is in fact new), or is it more likely to have been caused by the dripping faucet we've had forever and have neglected to take care of (meaning the stain was already there)? Or some combination of the two? And, finally, can anything be done short of refinishing the tub?
And yes, I now know that one shouldn't use bleach on the bathtub, but I'm still curious if a single application could cause something this serious.
Thanks for any advice; I'm definitely inexpert in this domain. Feel free to send email to ivanxqz at yahoo dot com (obviously subtitute "@" for " at " and "." for " dot ".
Best Ivan.
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Rust stains are common in areas with well water or with water that contain a lot of iron. Any of the rust remover cleaners (The Works, CLR, LimeAway) should remove it, although the stain will most likely return. You may have hard water, and is water softener salt specifically formulated to remove iron. It is fine to use (diluted) bleach on porcelain.
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As Phisherman says, bleach shouldn't be a problem on porcelain. Porcelain is very a very inert, glass-like substance. The stain is from the water. If you think about it, the cleaning person cleaned the entire tub with bleach, but you only have stains near the drain and faucet. The glaze may have been removed or pitted near the drain from years of scrubbing or prior use of drain cleaners. A rough surface will stain much easier than a smooth one. In any event, I would use a rust remover like NOW, Zud, CLR, or Rust Out.
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