Brief Answer:
irritable bowel disease or inflamtory bowel diseas

Detailed Answer:
Hello,
Thanks for your query.
I understand your concern.
there are several possibilities for alternating constipation and diarrhea
few are
1 irritable bowel syndrome
2 inflammatory bowel diseases
such a long period of 20 years with alternating constipation and diarrhea i would first consider above two condition.
1 irritable bowel syndrome (ibs) : a functional GI disorder characterized by abdominal pain and altered bowel habits in the absence of specific and unique pathology.

Manifestations of IBS are as follows:

a) Altered bowel habits

b) Abdominal pain

c) Abdominal distention

Altered bowel habits in IBS may have the following characteristics:

i) Constipation variably results in complaints of hard stools of narrow caliber, painful or infrequent defecation, and intractability to laxatives

ii) Diarrhea usually is described as small volumes of loose stool, with evacuation preceded by urgency or frequent defecation

iii)Postprandial urgency is common, as is alternation between constipation and diarrhea
abdominal pain is usually diffuse .
there is no single study that can specifically diagnose this condition.

inflammatory bowel disease: there are mainly two variants in this one is crohn disease and other ulcerative colitis.
symptoms may be associated with inflammatory damage in the digestive tract

a)Diarrhea: mucus or blood may be present in the stool; can occur at night; incontinence may occur

b)Constipation: this may be the primary symptom in ulcerative colitis and limited to the rectum;

c)Bowel movement abnormalities: pain or rectal bleeding may be present, as well as severe urgency and tenesmus

d) Abdominal cramping and pain: commonly present in the right lower quadrant in Crohn disease; occur periumbilically or in the left lower quadrant in moderate to severe ulcerative colitis
there are several investigations available for the diagnosis of this condition both invasive and non invasive test.
this clinical details will help you draw a conclusion on what are you suffering from.
get basic investigation like complete blood picture, complete urine examination, stool test, ultra sound abdomen.
these are basic investigation that are needed for now.
further investigation might be needed after these like ct scan , endoscopy ,colonoscopy based on clinical condition and requirement.
hope i answered your query
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