Brief Answer:
Colitis / IBS / Specific causes diagnosed only on investigations.

Detailed Answer:
Hi.
Thanks for your query.

To recapitulate your symptoms: 28 / F - yellow mucus on wiping after bm - sharp pain when gas is trapped in lower abd - bloating - gas on sugar or flour - heartburn - nausea - persisted for 3 months despite diet change and probiotics...

Considering the symptoms, your age and the possible treatments you must have received from your GP (please mention any other treatment you have taken other than Probiotic and Prilosec), following are the possibilities of causes:
-Gastroenteritis with predominant colitis. Inflammation of the colon causing the yellow mucus, disturbed bacterial flora causing the gases and the pain thereupon; gases causing bloating which in turn cause pains as the bowel is already inflamed.
Intermittent nausea is a reflex of any gastrointestinal disturbance.
This causes intolerance to sugar, flour and other certain food and beverages.
-Irritable bowel syndrome- IBS : IF all the investigations you will undergo are normal, we can say this is due to IBS.
-Certain specific diseases like crohn's, tuberculosis and so on : can be said so only after the reports on investigations are suggestive or diagnostic of them.

I am sure the Specialist - The Gastroenterologist will take your detail history, examination and advise you investigations as per his findings. These may be tests of blood including that of sugar, Thyroid function tests and so on; stool and urine to start with. Followed by ultrasonography, CT scan of the abdomen and more important Colonoscopy and Upper GI Endoscopy. Certain specific tests to prove or rule out the diseases.

I hope this answers your query.
Please feel free to ask for more if you need to or if you feel there is a gap-of-communication.
My points are solely based on the history you have provided.
Wishing you a proper diagnosis and perfect curative treatment.

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