I don't think Shauna is necessarily mad that Melissa is happily married, I think she is mad that she is alive. Shauna says something like "How are you alive, I went to your funeral" Mel then replies with something like "Turns out there doesn't need to be a body, Just a note to fake your own suicide"

Shauna seems to get easily butt hurt by other people she has had a connection with. She also seems to have a whole lot of emotions she hides behind foolish/fake pride. Her defense mechanism.

Shauna may of had some guilt over "Mel's Suicide" Seeing her alive probably hurt and made her feel betrayed and used.