T.,
I could have written your question! My daughter is in first grade now and about to turn 7. Last year, (after 2 years of preschool) when she started kindergarten it was the same exact thing. Every single morning..."I have a tummy ache!". She is very, very smart but for some reason had major anxiety about being in school. I took her to the doctor several times and the doctor agreed that it was anxiety. She also started having to go potty (just urinate) during school...10-15 times a day. The doctor said that was also nerves after urine specimens etc.
It did get better toward the end of the year, but it was very frustrating because she LOVED school and her teachers! I think it was just being away from me for the full day as opposed to preschool.
Make sure you spend plenty of time with her when she is not in school, never ever be late picking her up (major anxiety!!) and let her talk about her feelings every day. I ask my daughter every day, "What was the best part of your day?, What was the worst part of your day?, What did you have for lunch? & What new thing did you learn today?" It gets her talking and she loves when I ask her those questions :) I would ask her about her teachers a lot too...does she think they like her? Are they "nice"? Is there anyone in her class that isn't being nice to her? And talk to her teachers. Tell them she is experiencing a lot of anxiety about school and ask them to be extra sweet to her. Also, make sure she is getting at LEAST 11 full hours of sleep every nite so being tired doesn't add to it.
My daughter admitted that she seemed to think she was going to get "in trouble" at school if she did something wrong. I don't know why she thought that, because she never did anything to get "in trouble" at home or at school but it made her afraid. Try to figure out what she is afraid of and address it.
Don't worry...it will get better! My daughter has only complained twice in first grade of a tummy ache in the morning and I knew what was bothering her. Good luck!