My 21 MOnth old continues to get sick. Any information appreciated.?

For the past three or four months my 21 month old daughter has been getting sick to her stomach every few weeks. Now she is getting sick more frequently. She has seen three different doctors and none of them can find anything wrong. They have done blood work, stool samples, x rays, and a number of tests. I am desperate. I need some place to turn I can't bare to see my baby hurt like this much longer any information or direction would be appreciated.