What happens if you exercise and you have tachycardia?

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.
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.