Why does hyperventilation cause symptoms?

When a person hyperventilates, extra carbon dioxide is lost from the lungs to the atmosphere, and when this happens, the pressure of carbon dioxide in the blood stream also starts to drop. Carbon dioxide is a prime regulator of all bodily systems and so low carbon dioxide pressure causes the body to malfunction in several different ways. Some of the ways that are likely to cause obvious symptoms include:

• A state of respiratory alkalosis is created, and as a direct result of this, less oxygen is released to tissue cells (Bohr effect). This means that the body does not have sufficient energy to operate as effectively and efficiently as it usually does. The person is likely to suffer from fatigue, lack of stamina, brain fog, and some kind of malfunction, where the body just doesn't work as well as it should.
• The nervous system becomes agitated, causing irritability, anxiety, insomnia, muscle twitching and restless legs.
• Histamine production increases, causing or exacerbating skin and airway irritations. Symptoms such as increased mucus production, swelling, itching, redness, rashes, sneezing, repeated throat clearing and scratching are common. 
• Smooth or involuntary muscles that are wrapped around practically every hollow part of the body, start to spasm. Examples of these hollow places are airways, blood vessels, and the digestive tract. When these tubes are constricted, the air, blood or food cannot pass smoothly along. Symptoms such as asthma, poor circulation, rapid or pounding heart beat, increased blood pressure, period pain, or irritable bowel syndrome may present themselves. Because less blood also flows to the brain, it is also harder to concentrate and to think.
• The large muscles in the legs gear up to run in preparation for the expected imminent danger that hyperventilation prepares us for. But if the stress that causes the hyperventilation is simply lying too long on your back while sleeping, then the skeletal muscles tighten, leading to cramp, or an overwhelming urge to stretch and move.

In addition to the lowering of carbon dioxide, hyperventilation also cools, dries and irritates the airways, leading them to become irritated, swollen and to produce extra mucus. These factors tend to narrow or block some part of the airway, making it more difficult to breathe lightly and quietly, perpetuating the problem.

The person is more likely to be dehydrated when they breathe too frequently, which is a big problem for the body since it is composed of approximately 70% water. Dehydration obviously causes an increased thirst, but it may also lead to confusion or an inability to think clearly, leathery, dry skin, and a tendency towards constipation.

The increased levels of the stress hormones, adrenalin and cortisol, rev up heart rate, blood pressure, blood sugar, and the appetite, with less calmness and ability to sit still.

None of the above are going to make you feel great, and as you can see in the section on the common symptoms of hyperventilation, there are many more things that can go wrong than these few itemised here.