Why does this happens to me?

See I am very clear good habitual guy and I am 20 year old I don't drink,don't smoke ,don't do any bad health habit except masturbating which is healthier but then also many of the time I get some kind of stomach pain head ache and some other problems where I have to attend a doctor to get rid of those problem So like this I have this ONE FOR PAST 14 DAYS I HAD BAD BOWL MOVEMENT WHERE IF I EAT ANYTHING I HAD TO GO FOR LATRINE (4-5 TIMES A DAY) 1 DAY LATRINE WAS MOSTLY BLACK(ONLY FOR 1 DAY) AND WITH THAT I HAVE ITCHING ALL OVER MY STOMACH,LEG,HAND ONLY.So I need to ask what is actually gone wrong in my body and where Considering three symptom itching,bad bowl,very little pain on right side stomach.and also some noise come in side my stomach like gad-gad-gadddd-gad-gad.

and do u people also get some little this kind of health during your life even if u have best health habit

There are many things the symptoms you describe could be but nobody can diagnose your medical problems over the Internet. Your best choice is to follow your doctor's advice.

If you do not trust one doctor, go to other's and get their advice.
If more than one doc is saying the same thing, than you know it's most likely right.

However watch out for mistake. You need to research medical yourself. Books and online to understand what the doc is talking about. Also if you give you an idea.

Also like to add, Rob is right. No one online can give you advice like that except a doc.

Get a second opinion from another doctor. Keep going from doctor to doctor until you get satisfactory answers and treatment. The symptoms you describe may be serious. Black bowel movements can be an indication of intestinal bleeding among other things. Definitely get checked out.