Beatrice comments, at the beginning of Act II:
"How tartly that gentleman looks. I never can see him but I am heartburned an hour after." (II, i)
She is saying her stomach is upset when she sees Don John because of the sour look on his face. From this we get further insight into the characters of both Beatrice and Don John - Beatrice is witty and funny, even about the prince's brother, and Don John clearly doesn't have a very pleasant countenance about him.