Are you a chest or diaphragm breather?
If you are finding that you feel dizzy when you are doing breathwork then it is likely that you are not breathing properly.
When you breathe properly you engage all your muscles of breathing including the diaphragm which is noticeable as that engages the belly area too. When you chest breathe then you are only using the accessory muscles of breathing, that is the rib muscles (intercostals) and scalene muscles in the neck. In order to fully engage the respiratory capacity of the lungs it is important to utilise the diaphragm which is the main muscle of breathing.
You can notice how you are breathing - the abdominal / diaphragm area needs to engage to use all the muscles. You can place your hands on your belly area and the breathing is not harsh and noisy if you use all the respiratory muscles - if it is then it's likely you are only chest breathing.
Also you need to be aware of your pace. Efficient breathing is usually silent and calm and does not feel strenuous. If you breathe too fast and frantically then you may end up exchanging out more CO2 than you are making which can change the acidity sensors in the body. The body always tries to bring itself to balance and dizziness can indicate you need to change something. Breathing is best if you feel comfortable.