Bf hanging w/ his EX

okay so i have a “thing” with this guy and we are planning on making us official soon but i noticed that he still talks to his ex quite a bit... She started commenting on his instagram posts and there was one specific one that caught my eye... and it was the fact that they were hanging out soon!!? personally for me i freaked out because i do NOT think that is okay at all. i asked him about it... and he said he “had” to hang out with her because they were in a friend group together but they are all going on a 4 night vacation all staying in the same cabin. So obviously i freaked out cuz i don’t want my boy staying the night with his ex!!(especially for 4 nights) am i wrong to freak out about it ?