Should I Try and Find Out What's Wrong?

I think open communication is best. I think it would be best for the both of you if you talk this out (because you sound like a great mother, who empathizes with your daughter when she is not feeling good, and you too are feeling stressed over this). She should appreciate the caring effort made by you, and it may be just what she needs to feel better. Now, if after you bring this up she becomes stand-offish, upset, or aggravated, THEN I would back off and give her some space. You obviously have a close relationship and know her well to know this habit appears when she is upset, so I would say talk to her!