I've been diagnosed with GAD and also have stomach issues and I tend to confuse needing to poo with the feeling of being anxious, and it's hard to tell if one causes the other, etc.
Also, now that I'm aware of this, I picked up on people telling me I seem nervous (i.e. stimming or doing tics, being restless) and I didn't realize it because I only "know/think" I'm nervous when I have to use the bathroom, at which time I may not really be nervous at all.
My mom gets angry when I take medication for anxiety when I have to go to the bathroom, say it doesn't work, and then go to the bathroom and I'm not "anxious" anymore.
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Diagnosed with Aspergers, ADHD, Bipolar Type II, OCD, and generalized anxiety.