I found this post in a dcurbanmoms forum thread called "Kindergartener doesn't want to go to school anymore."
My DS also started to dislike school in K and it got progressively worse as most of the other kids started to read and write with what must have seemed like ease in comparison to his struggles. It took until the end of 3rd grade to get the learning disability diagnosis and put all the pieces together. We thought there were social issues or something else going on but it was all frustration at having to work so hard to read and write and feeling stupid that he couldn't keep up with the others in class. He just couldn't explain to us that this is why he hated school until he was older so we got lots of vague complaints instead. The tummy ache complaint was a classic. I'm sure his tummy really did ache, but from anxiety about what was going to make him feel stupid in school that day, not from an actual physical issue.
Sounds just like last year with Younger Daughter!