Medicinal sleeping pills problem?

I have some medicinal sleeping pills ordered in from America to help with my sleeping cycle. They are a melatonin dietary supplement. All it contains is 3mg of melatonin per tablet, rice flour and gelatin. However, just 1 of these pills seems to put me into a very intensive REM state of sleep. Taking 2 gives me night terrors and bad dreams. Taking 3 or over causes extreme paranoia and very vivid nightmares throughout the night. I also seem to have increased nightmare rates since taking my first pill. I have had no confirmed sleeping disorders from a doctor before, but I do tend to sleepwalk. Is there any reason why these pills affect me like this? And is there any way to reduce or cure these effects the pills have caused? Thanks.