Psychosomatic symptoms don't quite work this way. For example sometimes you feel stressed at work and get a tummyache--that tummyache is real, linked to your stress response, and technically the stress is in your head. The person didn't believe they had IBS and suddenly develops a tummyache, they were stressed and had a physical response.

My Crohn's Disease is triggered by stress a lot of the time. Stress is caused by things that are "all in the head". My Crohn's Disease is therefore all in the head and I shouldn't worry that my intestines are completely fucked and I've thrown up my own shit before due to how fucked they are.