I'm new to all of this, and would like to know where I can ask a question regarding my own solution to a problem, which is from a contest that is already completed.
1) Is that allowed here? 1a) Is that allowed if the contest was not a Codeforces contest? (AtCoder in this case) 2) Do I ask it somewhere else?
You can just ask any problem in the comment section of the contest (if it is from that contest) or even write a blog on it. It does not necessarily have to be from Codeforces itself. Here, we discuss the problems with the community.
Wish you a great time on CF (Codeforces) ;)