Hi K.,
You know, it could be so many things, so the best advice is to have her see a doctor - although I truly do understand the problem of not having health insurance.
Are you in Oregon? If so, here's a link to the Oregon State Children's Health Plan - your daughter may qualify: http://www.healthinsurancefinders.com/healthinsurance/ore...
All that being said, when I was a child, I got periodic stomach aches that I later realized were due to stress.
So, it's entirely possible your daughter's stomach pain is due to stress (although, please have a doctor check her just to rule out something physical - you'd never forgive yourself if she did have something serious going on).
If it is stress, teaching her deep breathing techniques and doing those with her in the morning and in the evening will help, as well as warm baths where she can relax her body and breathe deeply into her belly. Tell her to breathe in a beautiful warm light (like gold or yellow) and imagine it going into her belly. Tell her to imagine the muscles in her stomach relaxing and opening. If you do this with her two times a day, within a few weeks, she will be able to quickly do a relaxing breathe at any time when she needs it.
You could also encourage her to draw how she feels or how her stomach feels. Seeing a child therapist would be good, of course, but that would cost money. There may be free or low cost options through the school or other avenues - other moms here may have good advice about that.
By the way, please don't be hard on yourself if it is stress. My parents were and are wonderful, loving people. The stress I experienced had to do with problems I had outside of my family. Life can be tough, even on our little ones, and we parents can not protect our children from everything.
Best of luck to all of you. M.