To be honest, I'm not entirely sure what this sentence is supposed to mean. What idea did you have after doing your homework? I do believe you need to clarify what your idea was about because otherwise it sounds clumsy to my non-native speaker ears.
Because the past progressive is made with was/were + -ing, and the negative uses wasn't/was not or weren't/were not. We use didn't with the past simple (went/said, etc become didn't go/say, etc).