Am I being jealous or simply I lost my self-confidence?

So I'm trying to get over a girl and it seems a dude pops up. And she's very chatty with him, although he's a little bit detached. Next thing I know I start comparing to him. What does she see in him etc etc. Last night I dreamed he kissed her. Am I being jealous? Have I lost confidence in me since she rejected me? Any suggestions.. :(