Something similar happened to me at a show once. Ironically also on a horse named Jack! The first class I was in was basic seat (aka equitation) and he was carrying on the entire class. Whinnying, fussing, being difficult in general. Fortunately he was a flat moving appy, and I'm used to big moving warmbloods, so I ended up 2nd in the class, since he appeared so difficult and I rode him through it well. There was about half an hour until our next class (english pleasure) and in that time he settled down and relaxed. Went into that class and he was a doll! I ended up with first in that class because he behaved so well!
But good job! 2nd is great for your first time in a class, especially on a dramatic pony ;)