Camel Pose (Ustrasana) is one of my favorite backbends. It has the potential to help us find tremendous openness in the thoracic spine—the upper back—where many of us are relatively tight. Most people tend to be more mobile in their cervical spine (neck) and lumbar spine (lower back).
Left to our own devices, our bodies will always do what is easiest and avoid doing what is hardest. This means that many people tend to backbend in a way that reinforces mobility in the neck and lower back (often creating hyper-mobility), while allowing the thoracic spine to grow stiffer. Camel Pose presents a wonderful opportunity to address this imbalance, but for many practitioners it actually exacerbates the tendency to do all the work in the neck and lower back. You may experience light-headedness because you haven't found a way to activate your thoracic spine and are therefore dropping your head back in a way that creates compression in the cervical spine.
My recommendation would be to practice Camel using blocks. Set them in their most upright position, to the outside of each of your ankles. Keep your hips stacked over your knees rather than letting them move forward (which is the body's attempt to avoid bending in the upper back by collapsing in the lower back). Lift your sternum and draw your shoulder blades in and up to create as much lift and openness in the chest as possible. Maintaining this intention, release your hands back to the blocks. Emphasize the action in the shoulder blades to create the bend in the upper back, and only drop your head back when and if you feel a very active lift in the heart. You won't go back as far using the blocks instead of your heels, and this will help you direct the backbend so that it becomes more precise. This work to open the thoracic spine and mitigate the tendency to over-bend elsewhere should also have a wonderful effect on your other backbends, allowing you to distribute each bend more evenly along the length of the spine.