I had 2 bouts of bilary colic and was sent to a surgeon who found a 15mm gallstone. why did it hurt when he pressed.on my gallbladder?

That may: be a sign of GB inflammation. Were you asked to take deep breath while palpated the region? That is called "Murphy's sign." However, deep palpation itself may be uncomfortable without gallbladder inflammation, so your best bet is to talk to the surgeon about the findings. If they suspected cholecystitis(GB inflammation), it is unlikely that you would've been sent home without that info.