Not a happy bunny
Definition 1 of 1
Pro Tip 1/3British Origin
This phrase is widely understood in the UK but might confuse people from other regions. 
Did you hear about Alice? She's not a happy bunny since the party got cancelled.
Pro Tip 2/3Informal Expression
Use it in casual conversation, but avoid in formal writing or professional context. 
When the concert was postponed, my brother was not a happy bunny.
Pro Tip 3/3Emotional State
It implies a mild, often temporary state of dissatisfaction, not extreme unhappiness. 
Jill's not a happy bunny about having to work overtime.