Not exactly sure if you're referring to this, but the past tense aspect is already in the 'did'. If you ask a question in English, you have to make a construction with an auxiliary verb, and if there's no auxiliary available, you use 'do' instead. Any conjugation that is necessary needs to be applied to the 'do', then.
You say something. -> Do you say something?
You said something. -> Did you say something?