I Have An 11 Year Boy Who Was Diagnosed With

I have an 11 year boy who was diagnosed with strep and given medicine. They put him on a stronger medicine because he was not improving. He finished that medicine. He then again within a week had a re-occurrence They again prescribe the medicine he just finished and the strep appears to be improving however now he has been fatigued and is complaining of stomach pains. They tested for mono and it came back negative. I am wondering what the fatigue and stomach pains and overall just yuck feeling maybe coming from.