Within your inner ear you have a small organ system called the vestibular system. This system is basically made up of three tubular circles that contain fluid. The liquid moves when you move, telling your brain what position your body is in. When you spin, the liquid also spins and gains momentum. The liquid continues to spin after you stop. Imagine stirring a cup of coffee for a while - even when you stop stirring, the coffee keeps swirling around in the mug.
So while the fluid in our vestibular system keeps spinning, we keep on getting messages telling us we’re still turning around. Our eyes are trying to adjust for this movement but can’t, and are simultaneously noticing that actually, we’re standing (more or less) still. All these confusing and conflicting messages cause our muscles to move us off balance and our vision to get very mixed up - the combination of which gives us the feeling we know as dizziness! Your brain thinks you’re still spinning, so you continue to feel that everything is going in circles-until the liquid stops moving.