Am I overreacting???

So my SO is taking his mom on a trip to a tropical country at the end of the month to visit his uncle. He just tells me that when they are there his female friend (who I've NEVER met nor have I ever really heard too much about until now) is going to be staying with them while they are there because she lives there or whatever....

Sorry but I'm actually soooo furious!!!! I just went upstairs because I don't want to fight around my daughter... and I can feel myself going into a rage so I just want to be alone.

Am I wrong to be so mad?? Like I know his mom and uncle will be there but still.... the fact of him having a great ol' time in paradise with another women makes me want to break things! Lol