There are many things that can be going on when you feel like this. If you have been out too long in the heat, you may have heat illness or dehydration. You may have a virus or your thyroid levels might be off. Some women feel like this when they are going through menopause. Always seek medical help right away if you feel dizzy or like you might pass out.
While the list below can be considered as a guide to educate yourself about these conditions, this is not a substitute for a diagnosis from a health care provider. There are many other medical conditions that also can be associated with your symptoms and signs. Here are a number of those from MedicineNet:
Dehydration is the excessive loss of body water. There are a number of causes of dehydration including heat exposure, prolonged vigorous exercise, and some diseases of the gastrointestinal tract. Symptoms of dehydration include headache, lightheadedness, constipation, and bad breath. Treatment for dehydration is to replace lost fluids and electrolytes.
When sleepiness interferes with daily routines and activities, or reduces the ability to function, it is called "problem sleepiness." A person can have problem sleepiness without realizing it. Symptoms of problem sleepiness include: consistently don't get enough sleep, or poor quality sleep, fall asleep while driving, struggle to stay awake when inactive (like watching TV or reading), have difficulty paying attention or concentrating at work, school, or home, have poor performance problems at work or school, have difficulty remembering things, have slowed responses, have difficulty controlling your emotions, and/or if you have to take naps on most days.
Arrhythmia (Irregular Heartbeat)
An arrhythmia is an abnormal heart rhythm. With an arrhythmia, the heartbeats may be irregular or too slow (bradycardia), to rapid (tachycardia), or too early. When a single heartbeat occurs earlier than normal, it is called a premature contraction.
Low Blood Pressure
Low blood pressure, also referred to as hypotension, is blood pressure that is so low that it causes symptoms or signs due to the low flow of blood through the arteries and veins. Some of the symptoms of low blood pressure include light-headedness, dizziness, and fainting if not enough blood is getting to the brain. Diseases and medications can also cause low blood pressure. When the flow of blood is too low to deliver enough oxygen and nutrients to vital organs such as the brain, heart, and kidneys; the organs do not function normally and may be permanently damaged.
At MedicineNet, we believe it is important to take charge of your health through measures such as a living healthy lifestyle, practicing preventative medicine, following a nutrition plan, and getting regular exercise. Understanding your symptoms and signs and educating yourself about health conditions are also a part of living your healthiest life. The links above will provide you with more detailed information on these medical conditions to help you inform yourself about the causes and available treatments for these conditions.