Dear Doctor, Since Last 4-5 Years I Am Suffering From

Dear Doctor,
Since last 4-5 years I am suffering from some stomach problems.
I am a vegetarian, male of 32 years age.My problem is that I suffer from random dysentery, vomiting tendency, abdominal pain etc. I observed that It happens all suddenly.
My doctor usually gave me the following--
1.Tab Normaxin (twice a day)
2.Cap Velgut (twice a day)
3.Tab Supradyn (Once a day)
4.tab Mirtaz 7.5 (afetr dinner)
After taking all these for 3 months I started feeling well. But the effect did last only for 2 months. Again the same problem started to affect me. Then last week doctor changed the following medicines--
1. Tab Refagut 400mg(twice a day for 2 weeks)
2. cap Providac(LACTOBACILLUS twice a day for 8 weeks)
3. cap reconia G(once a day for 4 months)
4. tab mirtaz 7.5(after dinner)
Now I am taking these medicines. But relief is still random. One day I feel better but the very next day uneasy in stomach. Doctor says that it is due to stress on intestines. But he cant explain the exact reason behind this "stress on intestines". While I have a very normal life.
I am completely clueless about my problem. Please help me.