Cactus Flower Murder Case

A murder was committed at 7:30 pm on Monday. The discoverer of the body was Ayumi's dentist, who was checking Ayumi's teeth at the time of the killing. Further inspection showed that the dentist had a good reason to kill the victim. But she had an alibi, so the police turned away their suspicion of her. Conan thought that there was a trick. What was it?