Don't be disheartened. Two cupcakes are not the end of the world! What kind of cupcakes were they - do you know how many calories they were? Did you incorporate them at all into your day or not (i.e. for a snack)? If I were ou, I would work out how many calories they were and then try to recoup those calories by doing a little extra activity every day until your next weigh in. Then you will still see the same results as you would have had you not eaten them.
Seeing results will take time, but doing it slow and steady decreases the chance that you get fed up and quit the diet and increased the likelihood of keeping the pounds off. Just take it one week at the time, and you'll soon start to see the difference. I am trying to lose 33 pounds, and if I was losing at 2lbs per week I would be at my goal weight had I started in the end of November. That is not really that long ago - somehow 16 weeks into the future seems further away, IYSWIM.
Chin up, dust those cupcake crumbs off it and start fresh tomorrow!