Most Helpful Girl
If you and her are still in some kind of contact, I would ask her why she unliked your posts. If you are prepared for the answer, good or bad, you can ask her even if you're not in contact much, either way you need I know because it will constantly play on your mind. Even though it is something as little as a social media post, it still gets to you because you have all these thoughts running through your head like 'what made her think to go and unlike them', 'does this mean she doesn't like our memories', I am in exactly the same boat with the photos on social media. It's almost like (for me anyways) they want to erase any memory they have of you both. Having said that, with the way I think, I personally don't see this as a negative. Sometimes people change things around because of guilt, maybe she's upset to remember the memories you guys had, and (in girl world) we do this because we are maybe frustrated and upset, so it's our way of 'taking it out on something' rather than actually taking it out on you, if that makes sense?
I would honestly just ask her why she's unliked then, why has she gone through all the effort to go through them an unlike each one. Another thing us girl do, is things like that for attention. Good luck and hope this helps a little :)