I think she suspects, but does not want to assume given her lone at the end she clearly assumed Qi Nian did it to become emperor, but realises after saying it that it would make no sense to have such a dangerous and zealously garded secret purely for that. She might have also realised the empress dowager would have to know this and as such realise there is more to this than it seems from what she knows and instead reaches out to Qi Nian to give "him" the chance to speak and explain "himself" which is an extension of her saying
And
With the proverbial gun being fired when she says
Where she seemingly realises two things
- There is too much risk for Qi Nian to try doing this, hence her asking if "he" wanted to be emperor that bad, as there would be almost certainly people who would find out or know including
- The empress dowager having to know this given she gave birth to "him" and previously being established they have a strained relationship with the empress dowager clearly being controlling and manipulative towards everyone
which is almost certainly why she stops herself from assuming and reaches out as it clicks this was likely a forced thing: yet, she does not understand yet why