Dehydration typically does occur when either you do not drink very many liquids or you have excess loss of liquids that you cannot keep up with by drinking - for example, from vomiting
or sweating. Symptoms of dehydration vary depending on how severe the dehydration is, but they can include things like dry mouth, dizziness, and weakness, as well as racing heart rate.
If you are currently having symptoms like this right now, you need to get in to see a doctor
right away. If you feel extremely weak or dizzy, or if your heart is racing, then you need to go to the emergency room for immediate treatment and not wait until you can make an office appointment with your doctor.
It is hard to know why you would keep getting dehydrated without knowing more about what is going on recently with your health. However, your doctor will be able to evaluate you for signs of dehydration and for evidence of any obvious cause of dehydration (such as a viral stomach bug causing nausea, for example) and treat these causes as needed. If no evidence of dehydration is found, then your doctor will still want to investigate why you are having these symptoms.