My girlfriend isn't so talkative anymore. any ideas?

Anonymous
i texted her to bring me something at 9:00am (almost 4 hours ago) that i forgot at the house because i didn't have time to go home and get it myself and she has the day off. when she came to my work, it looked like something was bothering her. i couldn't ask her if something was because she handed my stuff to me and then turned around and slowly walked away after i took it from her hand and said "thanks" and she said "yea, np" with a slight but disappointed grin. i came home for lunch and she's just been really quiet. i've tried talking to her, but i can just tell she doesn't want to or isn't in the mood to talk. she doesn't look tired either, which could cause that. i really want to know if something is bothering her but she doesn't seem in a talkative mood. i'm sitting beside her on the couch now because that's where she is and she just doesn't seem like herself.

usually if i sit by her, she comes closer to me like "automatically" (she's always in a good mood). when i sat beside her like not that long ago, she just stayed in her spot staring blankly into her own world it seemed like until i said her name a few times to get her attention.

i go back to work in 30 minutes because I'm waiting for something to be done and it will be finished by then. please don't answer this with she's being unfaithful, treacherous, disloyal, etc because that is not helpful to this situation at all even though it's the first things to assume in relationships.
My girlfriend isn't so talkative anymore. any ideas?
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