Obviously, if she's complaining of pain, take her to the pediatrician in case there's a physical cause. If after your best efforts you are still clueless about why she's so stressed, you may want to see a child psychologist or psychiatrist. To keep anxiety in check, teach your child to use her imagination. Before a stressful situation, have her imagine she's doing her favorite activity or is in her favorite place. Role-playing can help too.
Exercise is a great stress-buster if it's approached in a noncompetitive way.
As much as possible, try to approach difficult issues directly. When your child's dog dies, it may seem loving to tell him that Bootsie went to live on a farm, but he'll find out the truth better the information come from you so you can help him work out his feelings. Ditto for divorce, death, and so on. At times you may have to do some soul-searching. You are the door through which your child enters a very large world.