The exact same thing happened to me in college. I explained some statistics to my classmates and when they took the exams, they got a better grade than me. I don't know if it was the nerves or that my teacher, who was pregnant at that time, always demanded more from me than the other students, mistreated me in front of my classmates and because of her condition I didn't respond to her. However, at the end of the statistics course I managed to pass the subject, but with a very low grade. Sometimes I think that the teacher's pregnancy had a big influence on that, and she didn't like me.