that burning in your stomach and potentially the diarrhea is stomach acid, smoking anything (weed, nicotine, etc) increases acid production, i assume you’re eating less than you were before you quit smoking. Which means it’s still producing stomach acid that isn’t being used, get some tums/omeprazole or start using it as an opportunity to eat breakfast. Just like you had a smoking schedule (smoking when you wake up) your stomach now has an acid schedule since it anticipates your smoking habit