Taking too big of a sip hurts after surgery, but fortunately only for a couple of seconds, then it eases as the water trickles into your tiny tummy.
Ever swallow a big gulp of water or a bigger than usual bite of food without chewing it properly? That's exactly the pain - it just takes a smaller amount o trigger it.
A different thing is some people finding that plain water doesn't "sit well." My surgeon didn't give a reason, but said it's very common and suggested adding a bit of acid to the water, like a bit of lemon or lime juice or some Crystal Light.

Highest 303.4, Surgery 263, Current 186.8, Doctor's goal 180, My goal 150