Tests

Why is it that I think I did well on test, but really I totally bomb them?
For example, I studied and studied for a Health Science test, but in the end I ended up with a C, a C! When I was taking the test I thought I was doing pretty well, like I knew all of the questions. What happened? Maybe there's something wrong with me like bad luck, or something. Well, this has happened more than once this past school year. Maybe I'm just destined to not do well on tests. I should just work on my studying and take a shot at what ever comes my way and do extra credit in the mean time.