You see the answer very clearly in the flashback that Ellone gives you when you escape from the space-station.

In that flashback, you will see a few things that happened, like Rinoa forcing Irvine to go back, or Rinoa asking for the ring of Squall at Zell. And finnally you will see the scene right after the battle with Edea at the Galbadia Garden. In that scene you will see, how the "spirit" of Ultimecia enters Rinoa. Rinoa will instruct Seifer and then she gets into a coma. So Sir Bahamut is right. Rinoa became a sorceress after the battle with Edea in the Galbadia Garden. The reason that it seems that she gets to be a sorceress in space, is because then Ultimecia, knows her goal is nearby. She wants to free Adel. I think that Rinoa is actually still in a coma at the moment that she frees Adel. So, that's how I think everything is.