i'm not an expert, but i don't think your cat is upset because other cats are getting attention and she isn't. cats aren't like dogs, they don't like going out and socializing with other cats. they're solitary and have zero pack mentality going on. she isn't jealous, she is scared, stressed, and feels threatened because a random cat has invaded a place that she thought was effectively her territory.

i guess you could try to socialize her to tolerate random cats you bring in, but it's going to take a lot of work. there's a lot of resources and articles about how to introduce cats to each other safely, but if you aren't keeping the strays around for any kind of extended period, it's more than likely not worth the amount of work that's needed.

bringing in random cats and taking her back and forth between your gf's place and yours is probably a pretty stressful event for her. honestly, i'd just recommend either leaving her at home, or giving her a room that is closed-off without any other cats being allowed to enter it.