After 6 months, unless you're just not good at seeing the signs which are usually quite obvious, I highly doubt she has another boyfriend.
Here's my take:
75%: She wats to have her cake and eat it too. She doesn’t publicize the relationship because she wants to keep her options open just in case something better comes along. Women are famous for this and trade-up all the time.
25%: Some other crap like family issues or maybe even mental issues. Having a hard time getting my arms around this one, but it's possible.
As
@Modern Man Advice said, don’t get snide or butt hurt about it. If it were me, I'd not mention her on my social media and change your relationship status to something more benign. Then, I would back off for a bit and see what happens. Get really busy. Fill your schedule and see how she reacts. If she presses you about it, downplay it and just say you've been really busy with s*it and leave it at that. It might get the hamster wheel spinning. Maybe you've made this too easy for her and doesn't value the relationship. Women who have to work to keep a guy and compete for his time are much more interested.
Remember the golden principle that women are only a small tasty side dish on your huge plate full of food--and they are certainly not the main course. Focus on you, money, career, skills, body. Don't give in to the quick dopamine hits goalless guys get by constantly chasing p*ssy to the detriment of other important areas of their life.
Ask yourself: Is she good enough to be with you? For me, any woman that "hid" our relationship, especially if exclusive and 6 months in, would be reduced to plate status only.
Good luck.
~Dash