Alpha’s Rejected Mate

Demetria is a runt in her back and well-loved. Her mother is the most feared warrior in the werewolf world. Her father is a Beta in an allied pack.
Today is the day she finds her mate. She is overly excited to see who he is. Her best friend Louie says it is the alpha since they have been good friends for years.
When she is at school, she finds out it is the alpha. They were together all day. After school, he came looking for her saying he wanted to be fully mated. When she gives her virginity willingly to her mate expecting them to mark each other. When they were done, she heard the worst thing possible. “I Justin Thomas future of the Red Moon Pack reject you Demetria Thomas as my future mate and Luna of the Red Moon Pack.”
What will happen when she finds out she’s pregnant and rejected? She moves to her father's pack so she would see him with other she-wolves. He all of a sudden wants her back. He starts to stalk her. How does she deal with her ex-mate?