DarkNemesis, it was probably your cats.
Ever be driving on the highway and a car is merging in front of you? Naturally some people will push the gas pedal harder when trying to judge the speed of the car they are merging in front of...but most people rarely push their pedal that much so the car is seeing rpms much higher than usual, this causes the cats to expel a bunch of crap and buildup. Since you're driving behind them you'll instantly smell a crappy smell as they merge in front of you.
'07 335i Sedan
'97 F355 Spider