There's not many feelings that can compare to the wave of emotions you're likely to experience upon learning your ex is seeing someone new just a few days or weeks after you broke up. It's a very painful thought, and one that can turn the strongest men into sobbing wrecks. But, while it may seem absolutely terrible right now, things aren't actually as bad as they seem.
And regardless of how painful it might be to know your ex is dating someone new , you absolutely should not say or do anything right now. If you try to ask your ex about this new person, you're almost certainly going to cause irreversible damage to your chances of getting them back. If you try to somehow sabatoge their relationsip, you're really killing all hope of ever winning your ex back. So, for the time being, just take this one on the chin and try to accept it as an unfortunate reality for now.
The number one reason you shouldn't be overly concerned that your ex is dating someone new is that this is almost certainly a rebound 'fling' for your ex. He or she hasn't fallen madly in love with someone new, and you haven't been 'replaced' in your ex's mind. It's just a rebound.