That's by design. Unverified and newbie members aren't allowed to create new tags, only use existing ones. You probably crossed the threshold between a newbie member and a community member (these are our terms for the permissions levels) in those three hours.
By the way, the criteria for promotion from newbie member to community member is 5 or more posts + 5 or more days of membership or 15+ reputation points. Judging by your stats, I'm going to guess that it was the post threshold that you hadn't crossed yet.