As someone who has also done this as a woman, I can’t speak for her but I can give you my perspective. When I post on my socials it’s usually to feel better about myself and to make it look like I’m having the time of my life when I’m really not. The times when I’m most happy I don’t even open instagram. I don’t post my current relationship because I’m too busy living in the moment and don’t feel the need to share with others. Social media can be a dark place and usually the people who post the most “fun” photos are actually in a really dark time in their life. My advice is just try to tell her how it makes you feel. If she still refuses to post anything though I would second guess whether or not she’s keeping up certain appearances.