Dog stomach pain. Advice.

Hello everyone! I have questions about my Male dog who has been feeling uncomfortable for the past 3 hours.

He didn't eat this morning but he ate his lunch (usual, he ate it clean) but right after he ate he immediately started to flex his stomach like it hurt. Like stretching his rear and can't seem to settle down. A little drooling. No panting, no fever, didn't vomit, he isn't pale too.

He's been feeling fine last night. He drinks water too. He just can't seem to lie down and rest properly.