The lactulose solution gave me diarrhoea, is this normal?

Some people get diarrhoea when they take the lactulose, but this is rare.

The lactulose solution for the SIBO test is designed to be metabolised in the digestive system quite quickly.

Any reaction people have is usually minor and doesn't last for long. The solution is not a known irritant: foods or pharmaceutical products are more likely to be a problem for most people than lactulose.