1. Is something on your mind?
你担心什么呢?
Do you have something on your mind?
What's on your mind?

2. Why are you so glum?
你怎么看起来闷闷不乐的？
Glum:忧郁的

3. Why the long face? （Why do you have such a long face?）
你怎么看起来这么不高兴呢？
Why do you seem so unhappy?

4. You're not yourself today.
你今天有点心神不宁。
You don't seem to be yourself today.

5. Who are you irritated with?
谁惹你生气了？
Be irritated with 的意思是“为……而急躁，为……而发怒”。