Hi Kat,
The fault you describe sounds like possibly an oxygen sensor problem, to which i have sent you a link to describe what the situation could be,
OR it could be the same thing that happened to my wife's car a while ago......a light came on which pointed to emissions.......i took it in to Honda's, under warranty......they put it on a diagnostic check and found no fault at all.......the mechanic mentioned to me if i used BP petrol, as it seems to be a common error that comes up on Honda's and other makes too, that they dont seem to like BP petrol....weird, i know........so he advised to change to Esso, Shell, Texaco, Tesco, Asda, Sainsbury's etc......anything but BP.....she did and no fault light for nearly a year now.
In your situation, i would get it diagnostically checked at a local reputable garage ( they may not charge you, if you regularily take the car there for MOT/Servicing etc ) and checked for a lambda/oxygen sensor fault, or other... to which will come up if there is one on your car....and take it from there.
Best of luck to you and hope it gets sorted out. :0)
http://auto.howstuffworks.com/question257.htm