I always floss at night & rinse with mouthwash every day. so why do i keep getting cavities!?

Wrong assumptions: You assume that just by flossing and brushing you are immune to cavities... Not so. If it was that easy, everyone would just brush and floss and there would be no need for dentists. Diet, genetics, prior dental care, and how well you brush and floss all play a factor in the formation of cavities.

Other factors : Beside how well you are flossing and brushing , there are other factors that contribute to getting cavities, like genetics, diet and eating habits and maintaining good oral hygiene

Cavities: First, mouthwashes don't prevent cavities and aren't even a necessary part of good oral hygiene. We can't tell how well you are actually flossing and brushing. It's also necessary after meals and just not at night. Are you brushing and flossing before or after eating? Your diet also plays an important role. Snacks? See your dentist who can evaluate your oral hygiene effectiveness, diet, etc.