My 5 Year Old Has Began To Have Stomachaches Frequently;

my 5 year old has began to have stomachaches frequently; he complained of a stomachache about a week ago and to him to ER and they took one look at his throat and said he had strep (although the strep culture was negative) she explained that it could be 50/50% false negative. while taking the antibiotics no stomachaches but he was been off of them for a few days now and first stomachache tonight. he also complained of an itchy anus while sitting on toilet while having the stomachache. i am not sure what could be causing these stomachaches