Girlfriend is going on holiday with her ex

This is my first post. My girlfriend of 8 months is booked to go to Amsterdam with her ex boyfriend for a weekend. She has been friends with him for a long time, maybe 20 years, and they were only together for around 3 months maybe 14/15 years ago. They both had long term relationships so kind of fell out of touch for a while but got friendly again when those relationships broke down. She says she can't remember if they had wed or not. Anyways, they had booked to go to New York over the new year to see a mutual friend but the trio didn't go ahead as he was sick. This had been planned but not booked before I knew her, and was booked about two months into the relationship. I wasn't that happy, but didn't want to come across as possessive and jaelous so told Her I wasn't ecstatic but I understood that she had a life before me and I was ok about it. She did say that she thought he might have designs on her but she wasn't interested and if he tried anything she would make sure to tell him she wasn't interested. The other day a few days after her birthday she said that this friend booked her a surprise trip for just the two of them in Amsterdam. He had never bought her a present before never mind a present like that. She told Me that this trip was booked and she was going to go. I asked her was it a twin or a divorce bed and she said she didn't know. I asked her to ask him and he told Her that it was a double as it was the last available at that deal. I said I wasn't happy with either the trip or the sleeping arrangements. She knew I wasn't happy but she didn't say that she would change the sleeping arrangements. She then told him that I wasn't happy and he told her he would change it to a twin room. She says he has since done this. I'm still not really happy about this trip and am wondering if I am being paranoid. I really love her and she says she really loves me and I do believe that's how she feels about me. But I'm still Not keen on my girlfriend going away for a few nights of drinking , probably drugs and partying with a guy she uses to be intimate. She didn't book it so I can't blame her for that, but I'm still considering my future with her now, and am wondering if I'm over reacting

Edit she said she couldn't remember if they had sex or not previously

When it's all said and done, your GF has a choice whether she goes on the trip or not. Her actions tell you that she wants/needs to go and spend time with her ex and friend of 20 years, and yeah, it's all OK that she didn't book it but that's basically beside the point. It's all very well that she had a life before you, but she needs to understand that it doesn't give her the right to exclude you when she goes to spend time with other people, particularly an ex, from her past. Given the occasion, her birthday, one would think she would wish to spend it with the guy she 'really loves' or at least, share it with other people from her past with him. Yeah, you may not want to come across as a jealous and possessive guy but your GF isn't really giving you much of a choice not to be.