What can it be if this month I didn't have a flow only blood clots, I get dizzy nauseous and lower back pains. Other periods are light w/ blood clots?

Anemia. Pain from cramping can cause whats called a vaso-vagal reaction; also need to watch for anemia, check upt but likely may need the birth control pill; clotting, especially if big, can be a sign u r developing anemia.