I never immediately assume an infection, it could be mini or bottle krausen, which people do see on occasion.
Carbing beer is similiar to fermentation in a fermenter...Yeast eats sugars and processes co2 and alchohol....Since ale yeasts are top fermenting, it works just like in your fermenter...Krauzens often get formed and then fall through to the bottom during the 3 week process... Some yeasts are more apt to do it more dramatically than others...and priming with dme sometimes makes it noticeable...
I actually have a theory that it happens all the time, but since a lot of us just stick our bottles in a box, in a dark place and forget about them for 3 or more weeks, (instead of staring at them obsesively), we never notice it occuring.
Let the beer go 3-4 weeks in the bottle and see if it doesn't fall, I'd only worry if it hasn't fallen after a month in the bottle...
I don't agree with the Edge's suggestion to open one early...A gusher or off flavor before a beer is fully bottle conditioned and carbed for 3 weeks, isn't all that uncommon...If you aren't familiar with the difference between a green tasting beer, and an infection, then how would you be able to tell?