A: How fast were you driving when the police stopped you?
B: I am not sure, but I wasn't driving very fast.
I think this sentence is correct but I am not sure, but I didn't driving very fast is correct because it is a true thing in the past.
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I was driving very fast.
This sentence is in the past continuous. To change it into the negative, you should add only "not" after the helping verb "was". So the correct sentence is:
I was not driving very fast.
You never use didn't before an -ing form of a verb).
You use the auxiliary did + not to form a sentence in the past simple negative. For example:
I drove very fast. (The sentence is in the affirmative)
I didn't drive very fast. (It's in the negative)