Would a movie be considered a sequel if it takes place after another movie but was made before?

As trilogies go Star Wars 4-6 are neither prequels nor sequels. They are the originals. 1-3 are prequels. 7-9 are sequels. Though an individual movie that follows a previous one within a trilogy would be considered a sequel. For example, Empire Strikes Back is the sequel to A New Hope... and so on.

They are sequels, but because 4-6 came out first then the new classification falls on 1-3 as prequels

yes they would be sequal,but on starwars...1-3 are called PREQUALS.

In terms of where they fall chronologically in the overall story, technically the original trilogy would be considered sequels to the second trilogy. But in terms of which films were actually filmed and released first, then the second trilogy would be considered prequels to the original trilogy.

Why didn't Lucas simply make the movies in story order instead of making the second half first and the first half second? Beats me. I'd assume it came down to budget, but while I grew up with the original trilogy and love those films, I'm not a Star Wars universe expert. 

No, 1 - 3 were sequels.

Yes.  It doesn’t matter when each episode is made, but where it sits in the timeline of the story as a whole.