Yandere Eren x Reader: The change!

Being Eren's best friend for as long as you can remember, you started to notice a change within him.
As soon as You, Eren, Mikasa and Armin was in the middle of the first year of Military training.

The changes were getting noticeable to the others as well, from:
Interrupting conversations with male trainees to threatening them behind your back!

Hmmm.... I wonder why?