My ex's new girlfriend

Tiffany
My son's dad and I ended our relationship about 5 years ago. We get along for the most part. Anyway, long story short, I just found out that he's been having his new girlfriend share the bed with him and our son. Our son is 9. He lives in a 2 bdrm trailer but his aunt is currently occupying what was his bdrm so he's been sleeping in our son's room with him. 
I think it's very inappropriate and should not be happening. She either needs to not spend the night during the two days my son is there or she needs to sleep on the couch. 
Am I overreacting? I truly don't feel that I am. My kid shouldn't be fighting for space on HIS bed in HIS room. She's rather large and it's a full size bed. So two adults and a child is a pretty tight squeeze. 
As far as I know this is my ex's first relationship since we broke up. He's a very negative person and is addicted to video games so I'm not surprised. I think he's being selfish by putting our son in that situation. 
Anyone else have thoughts on this?