What Causes Burning Sensation On Left Side At Waist Of Abdominal Area?

6 year old grandson complaning of lower abdomen area, burning sensation. Has legs in fetal position while lying down, cannot sleep. Last evening could not have bowel movement. This morning had a movement. Good amount, like a grown up, chocolate color. Ate breakfast, 7:15am ,snacks potato chips 11:30am, lunch 1pm. Lunch he ate half turkey burger(I cut it in small pieces), 1/3 cup rice. He had ketchup with burger. Water to drink. Also rice milk with breakfast, but I had to make him drink it. He has not had much fluid, does not want it. He has many allergies. Complaining of burning on left side at waist of abdominal area. Feels like he has to have a bowel movement, but strains and nothing or small pieces of waste come out. What is wrong. Parents on the way home, now. No fever. Did vomit after dinner. Says he ate too much