Hemorrhoids- People think you only get them from being constipated and straining alot to use the bathroom but you can also get them from having diarrhea alot and this would cause the blood in the stools bright red blood. This sounds like it could be whats going on with you because you said you see the blood after the diarrhea episodes
It's good that you are seeing a GI as this is their specialty. If the blood is dark and you say you have stomach problems it may be inflammation, or a bleeding ulcer, which is not serious. If the blood is bright red, at your age I would say hemmorrhoids. At your young age it's rarely is anything serious, plus, all your tests were normal. The GI will be able to tell you for sure and it won't be anything that can be healed. Since you've been trying things like Zantac and Prilosec, I'm guessing it's your upper abdomen which would be inflammation of your stomach lining (not serious) or a bleeding ulcer (not serious). You have youth on your side, relax and look forward to being symptom free soon. Take care!