The tense seems strange. I would either say "I just said" or "I've been saying", depending on the length of what you were saying.

You could say that, but I think it sounds a bit awkward. I think it'd be better to say, "Did you not heard what I just said?" Or even "Did you not hear me?" or "didn't you hear me?" The first is more polite in my mind, while the others are more informal and to the point. Hope that helps :)

I agree with Jacob. The thing is that the perfect form "haven't you heard" sounds unnatural to me in that context. You should definitely use the simple past "didn't you hear" AND "I just said."