Dizziness is usually from lack of red blood cells. Normal to occur at second trimester as your baby requires more blood to build organs and other body systems. So your heart working very hard to pump enough supply. Do inform your gynae, eat more red meat (beef, pork, duck) or can check whether gynae can issue iron supplements for you. And start exercising more - my gynae told me aerobic exercise can help my body build up more red blood cells (walking and swimming mainly)