How can i get rid of my running sickness?

hello. For almost a year i have had a seious runnning problem. Everytime i run a long distance( and sometimes short distance) i feel a terrible stinging in my stomach and feel like i’m going to throw up or actually throw up. I am pretty sure that it is the speed that i’m running and not the distance because this happens on short runs too. I have tried absolutely everything: chewing my food more, eating bettter foods before my run, and even heartburn medicines. I have already gone to the doctor and they have taken my blood and an x ray and they say that nothing is wrong with me. Everyone assumes it is just me growing but i disagree because this has been gong on for almost a year and it doesn’t get better or worse. I have no idea what i schould do now but i know that i can’t run the school runs anymore. Thanks for your help.