Boxer seems to be sick?

1st off my female boxer has a sensitive stomach, 6 years old, and is in a healthy weight and eats Canidae Dry dog food.  Today early this morning she puked up clear fluid and look to be a small amount of blood in it.  About 30 minutes later, she refused to eat, so I offered her some yorgurt with a smaller meal than normal.  She ate that  About 90 minutes later she puked that.  When I got home later she refused to eat again and pondering on what to do.  I know if I don't feed she will puke.  She can't ever have an empty stomach (that sensitive stomach thing) or she will puke.  What can I do, thank you for your help...