she's always felt that way towards him, it's just something that has to do with her past life so it makes sense we weren't aware of it at this point (especially because she's been mainly focused in the biggest goal of her new life, which is protecting her family -so she has put aside the rest-).
There are several reasons as to why Lippe+Duke act sometimes a bit distant, others they put too much attention on the other, why they get close in the way they do, etc. etc., but we'll have to wait for more chapters to discover that, and more aspects of her previous life related with her persona, relationships, etc., but the author likes to hide many things and the fan-service so it's gonna take a while, haha.