Respected user, hi I evaluated your query thoroughly.
*First of all remove stigma from your mind that you have low immunity 
  power at the age of 18.
*Treatment before 2 years has nothing to do with present condition.
*Follow simple easy steps in your routine  & see the difference with time :
- keep BE POSITIVE principle in the life , whatever happens its your inner
  strength matters to take you out from every worst condition in this world.
- have plenty of liquids to maintain your hydration.
- have diet rich in proteins, iron, multivitamins, minerals , anti-oxidants, 
  carbohydates.
- take more green leafy vegetables, cereals , pulses, legumes , yoggurt, 
  fibers , fruits  for  better digestion.
- do regular walking , exercises, 
YOGA at home without failure.
- avoid 
stress / 
anxiety .
- keep your weight, BP , Blood sugar & 
cholesterol levels under control.
- have sound sleep of 8 hours.
- keep fix timings of breakfast , lunch , dinner
- avoid prolong fasting / overeating
- avoid going to sleep for 2 hours atleast after lunch / dinner.
- avoid 
smoking / alcohol / abuse substances if any using .
- engage yourself more in your liked activities .
- help your friends who have same problem.
- if 
abdominal pain is cumbersome can go for 
Ultrasonography of Abdomen 
   & Pelvis .
 Hope this clears your query.
 Thanks for using Health Care Magic & keeping trust in our medical services.
 Wishing you a very happy healthy life ahead.
 Welcome for any health related issues in future.
 Regards dear take care.