What Causes Nausea While Bowel Movement And Tremors In Legs?

Hello Doctor, My son has had this difficulty twice within the last week. He is laying in bed, then feels almost nauseous and that he needs a bowel movement. While having his bowel movement, his legs then the rest of his body begin to tremor which continues throughout his movement. He also feels like he will vomit, but hasn t yet. The first time I thought maybe just a bad reaction to food, but now this has occurred again. Also earlier, both times while eating supper, he didn t feel well when he was eating so didn t complete his meal. What could this be???