Do you confront them and ask them what's wrong? Do you ignore them until they get over whatever is bothering them? Or wait until they come to you?
I work with a girl that goes back and forth between running to me as soon as I walk in to the office and gives me a big hug. Then the next time I see her, she will barely get out a "hello".
One time we were talking and she suddenly got mad. I like wth? That time...she said she was upset because my husband and I had went out for a few drinks, with someone we had just met ...and she had asked me several times to go out with her and I had declined her invitation. I apoligized...didn't know what else to say. Everything was fine after that...for about a week, now she back to barely speaking again.
lisa