I just got my semester grades.?
Talk to the prof. Ask him to recalculate. Profs are human and they know they can make mistakes. Or you may have made a mistake and the prof can show you that. Every school has mechanisms to fix errors like this, even after the grades have gone in. Ive seen no statistics, but I bet a few percent of grades get changed every semester. The sooner you talk to the prof, the better.