trouble in college.

When i was in high school, id get A's and B's. My first semester in college i got A's and B's as well. Second semester i failed a class, got C's, B and an A. Now my third semester i have Cs and an A. Whats happening to me, and i getting dumber? Im really put down and im even starting to think that Im going to go NOWHERE in my future because of my grades. Its really embarrasing how I study so hard for exams and my score is not what Id expect from myself. is there something wrong with me? for those who had grades like these, where are you i life now? Im starting to think im not good enough and im really upset.