gf getting together with her ex

we have been in a steady relationship for 4 years now but evrytime we see her boyfreind she changes her mood and seems to be less concious about me.we have spoken about it and i have conveyed my feelings to her, but i still feel the adrenaline rushing when she tells me(she does tell and that i apreciate) that he had called or that she met him.but it got out of hand when my parenst got a call from someone claiming to be her bf and telling he needed me to back off.i told her this and was shocked that she wudnt even consider about it.she says its imposible.but i do n wanna rule out any possibilty of whose out to ruin our relationship.am really confused and lost for words and totally in shock.just how am i supposed to react here?