It so easily causes a downward spiral. You get anxious because your stomach is off, and your stomach gets off because you're anxious.
Anecdote:
In my first semester at University, I maintained an A average, at a university where the entrance requirements were fairly lax but the first year was notoriously tough. I managed quite well, was happy and unstressed. I'd had a tough time of it in middle & early high school but switched schools and had a really fantastic final two years, and I was riding that wave of confidence.
That Christmas, I got norovirus. Hospitalized for dehydration during, just completely crippled for a week by flu-like symptoms. If you've ever had the flu and you know that hollow, ravenous-but-if-I-eat-I'm-going-to-be-sick-again feeling in the immediate wake of it? It never went away for me after that bout. I could be full and I'd still feel it. For about ten years, I was so sensitive to food that I'd have an upset stomach after one third or half of my meals, depending. Anxiety went hand in hand with it, and it was very easy to be anxious when my stomach was singing whale songs in class. Every excursion became a question of balancing whether I'd skip a meal and know that I'd be hungry with a gurgling stomach or eat and roll the dice re: having to miss the event/class/social activity because I was ill.
Went to a number of doctors that the university clinic referred me to, got the whole battery of tests, some twice, and in the end they just scratched their heads and couldn't give me any answers (except for tentative diagnoses based solely on process of elimination). At my family's urging I struggled to identify problem foods, and very carefully tried rotating various foods out of my diet, trying to calculate what worked and what didn't. Nothing really panned out. I tried probiotics and supplements and vitamins and fad foods like kale and tried being vegetarian.
At my lowest point, now plodding through University, I had anxiety bad enough that I'd sweat and have trouble breathing steadily just from leaving my apartment. If there were people walking down the sidewalk on the far side of the street who hadn't even looked in my direction and I heard them laugh, the first impulse, with near-certainty, was that they were laughing at me. I was a hair away from being a shut-in.
I eventually left university because I couldn't do it anymore. I broke away from existing patterns, I started a career that I love (am a fiction writer) and got away from the stresses of classes and moved away from the stresses of family to live in a quieter small town. 80% of my stomach problems went away. From one in two or one in three meals to an occurrence of one in twelve or so. I shifted my diet to keto, not even trying to address my stomach problems at that point, but just to lose weight, and the problems have vanished. I still have a slightly anxious mentality, but the anxiety as a borderline mental illness has subsided with the stomach problems, and vice-versa.
TL;DR: Got Superflu, in wake of it, chronic stomach problems, crippling anxiety where there was none before. Did eventually get a handle on it. Definitely believe in the mind-stomach and stomach-mind link.