I don't know about the serotonin connection, but I certainly think that my anxiety can exacerbate my digestive symptoms in a positive feedback process. In other words, I have had stomach issues produced initially from anxiety that then become the focus of the anxiety which then amplifies the digestive issues. At least that's how it feels, and when I can distract myself with other things, I can uncouple the process.
Remember also that anxiety can make your sensations heightened to what you're feeling in your body.
I don't know if any of this resonates with what you were asking.