Melatonin side effects?

[TheVolturi]

Hi all. My ds age 8 has asd and had been taking melatonin 2mg nightly for a few months, well almost 4 months. The other day he was suddenly in agony with stomach cramps which lasted over an hour. No upset stomach. And he's been getting regular less intense stomach aches that don't last as long. Something he's never had before. Also had quite a few episodes of nausea. Over the last month or so I would say mostly. After the bad stomach ache at weekend I stopped the melatonin for a few nights. He had it last night and woke up this morning feeling very sick.
Is it likely to be the melatonin?

Sounds rather strange as melatonin is (as far as I was told) not an active drug but an synthetic sleep hormone.
Have you spoken to your GP (or whoever prescribed it)
My now 13 year old has been on melatonin for about 3 years and I have not noticed any side effects on him (although he is non verbal so would not openly volunteer symptoms). He's never shown any signs of discomfort that would be linked to stomach pains.

My 4 year old son got more restless and hyper with the liquid melatonin. His paediatrician then prescribed 1mg prolonged release tablets.