Brief Answer:
Clinical evaluation and test for Breathing problems. Inter-related problems

Detailed Answer:
Hi young man.
Thanks for your query and an elucidate history.
Noted the history of pituitary tumor, its treatment and Cushing's disease now in remission.

Note your present problem related to breathing and bloating of the abdomen, which you have put in properly for me to understand.
You have problem in deep breathing, if you do there is swallowing of the air (Aerophagia) and the abdomen gets distended. This further increases the intake of air in lungs, increases the anxiety. Anxiety further increases both the problems.

This is sort of a vicious cycle.
The possibilities are:
Basic problem of anxiety. Anxiety causes both the problems of difficulty in deep breathing and aerophagia.
These (difficulty in deep breathing and abdominal distension) in turn increase the anxiety.
And the cycle goes on.

The solution to these problems can be:
-Get the clinical evaluation and physical examination done by a Physician / Chest physician. Tests of Pulmonary function, X-ray of the chest and other related tests as advised by Him. This will help to confirm or rule out the chest problem. If the problem is detected, it can be treated well and one problem will solve and automatically will reduce the aerophagia and anxiety.
-If the clinical examination and the tests related to the chest are normal, the basic reason obviously is anxiety then.
Proper treatment of anxiety by medications and self-control techniques will solve the other two problems.
-Distension of the abdomen, as you have rightly noted looks to be due to breathing problem and / or anxiety, the control of which will automatically solve all your problems.

I hope this answer helps you to get a proper diagnosis, and get a proper further treatment. Please feel free to ask for more or if you need further clarifications if you feel there is a gap of communication.

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Brief Answer:
GERD can not - yes, the Pulmonologist.

Detailed Answer:
No, GERD can not cause this.
In fact one of the reasons of GERD are all these three problems you are having.
Yes, the first step is to consult a Pulmonologist.

Please give the feedback.

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