Aiyoh mama, don't worry too much lah, one bottle of Yakult shouldn't cause what happened last night, but the tummy pain and restless sleep could be from either the Yakult (some kids' tummies are just sensitive to it), the potato texture, or maybe just something passing through his system. A few of my mum friends say their toddlers also react weirdly to random foods sometimes, so hard to pinpoint exactly what triggered it. Since he's sleeping now and didn't have fever, I'd say monitor him through today, if the tummy pain continues or he gets lethargic, better check with your doctor just to be safe, especially since this was unusual for him. For now, I'd probably skip the Yakult for a bit and stick to plain foods his tummy knows well, you know your son best, but better safe than sorry with these little ones!