"не в своей тарелке" - literally "[to be] not in ones/your/my/his/her [own] plate".
1. to feel discomfort.
2. to feel not at your proper place/condition.
3. to be not in the mood.
Proposed etympology is French "n'être pas dans son assiette" where 'assiette' now means 'a plate', but in past could mean 'position', 'disposition', 'mood' etc.