In season 2 episode 7 of Fullmetal Alchemist: Brotherhood, as Pinako and Edward went to the brothers' old house to dig up Edward's mother's corpse to see if it was actually her they transmuted, it starts to rain. In a particular scene, Ed throws up a yellow liquid which I guess was a nicer way to depict vomiting.
But why does he throw up?
As you point out in the question, they are digging up Ed's mother, which is traumatic in itself. Additionally, Ed is a child and they are doing this for a traumatic reason: to check whether the transmutation that took his arm and leg and his brother's body was actually of her. Vomiting is a result of the stress response which can come from things like anxiety and trauma. Hence Ed vomiting from digging up his mother.
