Always have nightmares after taking melatonin. Is it because of melatonin?

Melatonin helps me fall asleep but I keep waking up and falling back to sleep and sometimes have bad nightmares. I dont want to use sleeping pills and I melatonin works well, its just the nightmares that keep waking me up, im not even sure if melatonin is the cause. Any way to prevent nightmares?