Out of nowhere, your partner decides they need a new look. New clothes, eating habits and extra attention to appearance are indicators that something may be going on. If your partner never cared for exercise, but just signed up at the gym (and didn’t include you on their membership plan), it could be very innocent, but then again, sudden attention to attractiveness can indicate that someone may be on the prowl. It could be that they are concerned about their health, and in looking attractive to you… but sudden, drastic, positive, uncharacteristic changes in their overall look usually signal someone who is looking to attract; they may not be cheating yet, but may have already met someone they find interesting.
Suddenly he’s into the ballet, so why did he complain about buying tickets to see The Nutcracker last Christmas?