What Causes Irritable Bowel Syndrome?

Gud mrng doctor, iam 20,wt 54, my problem is,the last part of my stool will be a bit loose whereas initial part will be a bit constipated,frequency-once daily,i am suffering from around 4 yrs approx.,my gastroenterologist suggested celiac test,colonoscopy,endoscopy.....all reports were normal,and if i eat a bit large amount(or)eat spicy food(or)drink milk i get loose motions,i am having hypothyroidism,and osteopenia,sir i feel very tired and exhausted in starting hours of the day itself,it is having severe impact on my quality of life..please suggest me what should i do