What happens if you exercise and you have tachycardia?

Supposed to: When you exercise, your heart rate is supposed to go up. This is what happens when your heart functions correctly (tachycardia = high heart rate). If by your question you mean that you have supraventricular tachycardia, an arrhythmia, or some other preexisting heart problem, then the answer to your question depends on what the problem actually is. You should discuss this with your doctor.

Normal: It is normal for your heartrate to go up after exercise. It would be abnormal if it goes up too quickly or abnormally high, is irregular, or doesn't come down quickly enough t.

Exercise: When you exercise both your heart rate and blood pressure increase, usually the heart rate first then the blood pressure. If you have a history of an arrhythmia, then your heart rate can increase during exercise, or more commonly after exercise.