My best suggestion would be to sanitize a measuring cup and scoop a little out and take a gravity reading. If you're still above your target FG, it's probably just fine. Who knows why yeasts do what they do. Chances are, if it's been sealed the whole time, that nothing new has gotten into it, and the amount of yeast generated during the first 5 days of fermentation surely outnumber any amount of wild yeast you might have.
There's always a chance you have some sort of infection, but I'd bet you're just fine. Relax, etc etc.
Plus, there's always the chance you've just found some awesome local Lambic-style strain, and you can harvest it, and make a great holiday beer every year!!
Make beer, not bombs.
(...and not bottle bombs either.)
Primary Fermentor - *empty*
Primary Fermentor -
Secondary - Mystery IPA
Bottle Conditioning -
Keg Conditioning -
Draft: - Apfelwein, Belgian-style Dubbel w/ Grains of Paradise
Bottles:(and still in stock): FIRST AG - (I)IPA 4.1.07, PK's Warm Spring Marzen, Summer Cerveza -BANG, PK's Winter Spice II, Espresso Stout, Indiana Pale Ale, PK's Chocolate Bitter II, Porterfield Porter, and New Year's 2006 remnants.