[Sasuke]

I often read that just should be used with Present Perfect and that using it with Past Simple is incorrect.
Nevertheless, I usually find on the internet many people who use Past Simple with just.
So, to make an example:

A: Did you do what I asked you yesterday?
B: Yes, I just did it.

I imagine that this sentence is incorrect, isn't it?
So, all these expressions which are easily found here and there are American English or can also be used in British English?

Then, another and probably stupid doubt.
If in the question there is a tense I had better use it also in the reply, had I not? Or I can change if I prefer?

Example:

A: Have you seen the doctor recently?
B: Yep, I met him yesterday.

To sum up, my last question for today ():

A: I have just done what had been asked me yesterday.
B: I have just done what was asked me yesterday.
C: I just did what had been asked me yesterday.

Which of these three sentences is correct?
Can I use the past simple (case b) to express an action which happened before another or I have to choose the Past Perfect?

Thanks!