I noticed that a girl was checking me out and even flirting with me a lot, but now she rejected me. Why would that happen?

It can be confusing when you get signs of a possible new romance, but then someone's approach changes. First off, the most important thing to do is to not take her rejection personally. It's very likely that she was interested in you and wanted to get your attention. However, other circumstances might've come up. For example, her ex might've resurfaced in her life, or she may be going through a lot of stress and isn't ready to date. Respect where she's at right now and focus on how you can enjoy yourself, whether it's on your own or with a new romantic interest.