Taking Melatonin to help sleeping

Patient: I’m taking 5mg of melatonin to help me fall asleep every night. But every time where i take it, my mind and body will get intense for about 5 minutes. My body shivers and I have a hard breath until my body relax down. Is that normal?Also I heard that taking melatonin constantly will cause weight loss… is that true? I’m a 20yrs old male.