Why do I feel so bad for eating?

I am on a low carb diet cause I am 5'4 and i weight 147... I was doing great and then i had a huge bowl of cereal, and I am also always weighing myself..I feel so bad to the point where I want to cry. How long of a workout should I do to work it off..I want to throw it up so bad but i wont get into that habit again