She just likes your girlfriend more than she likes you.
I am totally just kidding! 
Welcome to the forums!
It may well be that she does get tuckered out playing during the day and is tired and ready to rest at your house (not a preference between her humans). Charlie and I run, romp, and play in the backyard but he is pretty quiet and just hangs out calmly in my recliner when he is inside the house. Not a bad thing, IMO!
If she is outgoing quite a bit of the time, and only a little shy at times, you can probably socialize her fairly easily. Have people quietly offer her some really smelly, yummy treats and later soft pats or belly rubs when she'll allow being touched. If she's being shy, she needs to be coaxed to initiate the approach to the human. She just needs to learn that almost "all" people are good (not just her two humans).
Hope you don't mind my joking with you a little bit.