In my current LTR my girl acts funny when other girls flirt with me.

Last week, a girl named S came to the desk I was working at and started telling me about her soccer game and other stuff. We were smiling and having a good 2 minute conversation while my girlfriend was there and after S left my girlfriend turned salty. My gf went from a smiling warm presence to having a disconnected and distant one. That continued until the night where she said "we should break up. You obviously like S more than me so go out with her. The way your eyes lit up when you were talking to her tells me that." I get her to stop tripping and we go to sleep.

Yesterday, my gf and I were eating lunch and everything was fine. S taps me on the shoulder and ducks behind the counter then we start talking again for about 1 minute. After S leaves, my gf starts acting funny again. We're walking back to our dorm and she's walking fast. Once we get to her room she hops on her bed and buries her face in her phone the whole afternoon till I leave to work.

That night, we're in bed making out and she just stops, turns around and says she's sleepy/not in the mood. It wasn't until then did I understand what she was doing.

My question is:

Is she doing this out of fear of me "leaving" with another girl (comfort test) or is it a shit test to see if she can act a certain way to make me stop talking to S (and other girls in general)?

Second question, How do I handle this? Do I call her out on it, act as if nothing is happening, or decide that I feel like I should sleep in my room for some bs reason to get her to know that I know what she's doing and don't accept it?

Apologizes if this sounds childish/elementary. I'm trying to understand what's going on and why the action I should take is the best one.