A robust porter should be too hoppy for most lactobacillus, at least the ones that make pleasing beers. It's possible it's pediococcus, which still makes lactic acid, but also a lot of diacetyl. It's more likely that you have acetobacter. How long was the barrel empty, how did you prepare it, and is it completely full?
The only thing worse than dumping beer is serving beer you should have dumped.