It's wrong to not believe in someone's pain.
Henry is talking with Janet when Janet doubles over and tells him her stomach is hurting out of nowhere.
Henry wants to address Janet's statement about being in pain.
Henry tells Janet he can go get her some pepto bismol from the store nearby if she wants.
Henry tells Janet she's being ridiculous and to stop being so dramatic, it can't possibly be that bad.
Janet is thankful that Henry is willing to go get her something to soothe her and asks him to do so.
Janet is annoyed on top of being in pain because Henry is being insensitive.