Every night i go to bed with moderate stomach pain and slight nausea. during the day i'm fine. what could be the cause?

Gastroparesis?: Patients with delayed stomach emptying often have worse symptoms over the course of the day as each meal piles on top of the last. After a prolonged fasting period while sleeping, this could abate. It's not a typical history, but symptoms such as fullness, upper bloating, inability to eat a full-sized meal, or vomiting undigested food several hours after eating would support this diagnosis.