18y female healthy weight/ good diet. hot flushes and dizziness with fatigue before previous 2 started. what could this be?

Before previous 2: Periods? This could be related to premenstrual syndrome if the symptoms only occur then. Please seek gynecological eval if symptoms are not resolving. Take care.

Autonomic Nerves: Dehydration, low blood pressure, low iron, low blood sugar, and sensitivity to changing hormone levels can all cause autonomic nervous system to react erratically right before period. Eat 6 equal meals with lots of fresh vegetables, high quality iron rich proteins, get a good night sleep ( often a problem right before period) and move through the day. Try jump roping. Evening primrose oil helps.

Started?: You state before previous two started. Are you referring to the fact that you have the symptoms before your menstrual cycle starts? If so these symptoms can be normal in the premenstrual phase. Make sure you're eating regularly that your blood sugar isn't too low. Also make sure to stay very well hydrated. These two things should help.