My son is 3.7 yrs old.since 2 days he had constipation and complained of stomachache and vomited.today he is having diarrhea .not watery but bright yellow colour. He did only once .lost his appetite. Also his weight is 13.5 kgs and height is 101cm. He is always below average for weight.birth wt was 2.9kgs. I also want to add his stool is mostly unformed .does once a day and is active. Pls tell me is there any problem?