Why are my statistics grades so bad?

I have always been a good student, even when I didn't study in high school. I took 6 years off after high school (Army) and returned to university a couple of years ago. I'm a junior now, with a 3.68 GPA. Math isn't my strong suit, but I studied hard and made it through my 2 college math classes with B's (I can't afford many more B's as I want to keep my GPA up for graduate school). I am taking my statistics class now. I LOVE the class, the material, the teacher, etc., have been doing the homework, the book homework, and the review for the test. We have had 3 quizzes, a major homework assignment, and a test. My first quiz was a 7/10. I figured I simply wasn't doing enough review, so adjusted accordingly. Second quiz: 6/10. Third quiz: 5/10 (after spending the entire day before doing homework and practice problems). Then I turned in my homework: 24/25. I'm baffled. Studied really hard for the test, was REALLY comfortable with the material, took the test and felt like I understood everything on it and what I was doing. I was very confident I had gotten an A or at least a high B. I got an 81. At this point I don't even know what it is that I don't understand. I do the review and practice problems with no trouble at all and feel as though I really understand that material. I'm baffled, ticked off, and pretty worried about the rest of the semester at this point. I went to my professor's office hours once to discuss it and she quizzed me over a few things and said she thought I was doing fine - that I completely understood the material. Has anyone else struggled in statistics or had this problem? How can I better prepare for or take these tests?