Girlfriend has a really weird lady friend

My(33m) s/o(26f) and i work together but it's a secret at work. Issue is she has a friend in the office who acts so weird towards me when we're not on good terms with my girlfriend eg bad looks,rude treatment etc but when we're on good terms she's okay and it's making me really uncomfortable and my GF still says that I'm imagining things, that there's nothing like that, that she doesn't tell her friend anything so I'm left wondering how the friend finds out we're not in good terms.Oh and our relationship(fairly new-3 months)is a secret between the three of us.I need any advice or input you guys may have cause it's driving me crazy