How can contamination have that much of an effect?
Wild yeasts/bacteria can ferment out more complex sugars than brewers strains can. It doesnt take much brettanomyces or whatever else to ferment out beer from 1.010 to 1.001. Thats quite a bit of co2 being generated in that little bottle.
But brett would put off specfic flavor i assume right? My beers don't have any off flavors besides being over carbonated.
Not necessarily. Time is a factor here. How long have they been in bottles? It can take months before you notice any real distinguishing characteristics from Brett. Also considering that its such a small amount of sugar its fermenting there may be negligible flavor effects, especially after a short period of time.
Say you do an all Brett beer, no yeast introduced, the beer will have Brett character at a young age, however the character of that can be more or less pronounced, or a completely different type of flavor all together, depending on the pH of the beer/wort.
If you do a beer fermented with a primary yeast strain then add Brett to the secondary intentionally the character of the Brett will be much more subtle.
Also consider that Brett is only one of many "bugs" and one of the more flavor pronounced ones at that in most cases. Other things like Lactobacillus dont have much flavor at all, they just sour the beer over time for the most part...taste a Berliner Weisse thats a month old and one thats 3 months old, big difference!
If I were you I would keep a bottle or 2 of this beer and tuck it away for a couple months, then try and drink some. You may notice an off flavor that isnt there now.