Ex's as friends

My SO of two years has an ex girlfriend as a close friend. She is constantly texting him and calling...almost daily. She even invited herself to stay the night at his house and they couldn't understand why I was mad! Just to clarify that...I made damn sure I was there that night. Every disagreement we have ever had revolves around her. He doesn't see anything wrong because they are just friends and is always sticking up for her. I recently met one of the ex's other friends and she told me that the ex does things to deliberately make me mad because she thinks it's funny. My SO is always sticking up for her no matter what I say. I am honestly at the end of my patience with this whole situation. Am I over reacting?