Welcome to to the world of airline pricing! This is a common occurrence, and part of the frequent flyer game. After all, if the price went up, you wouldn't want them to turn around and charge you more? This is a bit of a skill, and when you want to book a cheap ticket, you have to find out what the prevailing price is in that market and stay on top of it until the price drops to what you think is reasonable. With Delta these days, the best prices turn up 3-6 weeks before the date of travel, unless the route sells out, and then you're out of luck.