Thomazo, you've read my happiness explanation thread, yes? I think it'll help.
Those kinds of jumps are not technically supposed to happen, no, but they can, and it is because the new system uses dynamic variables (yields, etc.) instead of static ones like # of citizens. As such, the game recalculates happiness as those values change (I've been slowly working with this problem, trying to get the game to only re-evaluate happiness at the turn refresh, but it'll take time to do it right).
Were you doing anything specific when the change happened? That'll help me narrow it down also, if you think it is a bug, head over to the bug report thread, as it is easier for me to track bugs there.
G