Feels BAD. Dehydration can cause your to have headaches, fatigue, dry mouth, sore throat, achy muscles and joints. Concentrated urine. Constipation. Dizziness, nausea and fainting when you stand up --> heat exhaustion. Hot clammy skin..... Worse -> hot dry skin! Sign of heat stroke.
Thirst. Feeling very thirsty is a sign as is not peeing for a longer time than usual- your mouth may feel dry also Later as you become more dehydrated you may have cramps in your stomach or feel nauseated and also feel light headed.
Affects vital signs. When a person has severe dehydration usually she is dizzy, might have difficulty to be awake, have a high heart rate, low blood pressure.