I have a question

In the first special adapting the ending of the series, we see a conversation between Mikasa and Annie where Annie says she wants to enjoy her last moments while looking at Armin. Mikasa notices Annie blushing and looking shy, and Mikasa reacts with embarrassment.

My question is the following:

Is Mikasa's reaction because she interpreted Annie and Armin having sex, and that's why she asks, completely embarrassed, when it happened? (That's my interpretation; I could be wrong, and I'd like clarification.) Or is it simply a reaction to interpreting romantic feelings?