why is she acting like this?

So a couples months ago this girl started to work in my department she moved to the area for this temporary job with her boyfriend. She found out I liked someone there at work. We started to have lunch the 3 of us almost everyday. I got annoyed really fast because she started to fight with him and insult him yet she was always wanting to talk not letting me speak. I gave up and just started playing on my phone. I was gone for 2 months and every time I ask her about him she was very snappy. And last week she literally jumped in while he was trying to tell me something, its not the first time. I told her i had enough and to stop. She denied it and her response was very snappy. And said "he has a girlfriend" which has nothing to do with the issue if he wants to talk to me I don't see why she needs to but in. I feel like she said something to him and is trying for me to not find out because I see no other reason why she would act like this if she doent like him in the first place. Other piece of information til this day we don't know who is this girlfriend this guy is seeing. I have suspicious but they don't know each other.