I'm a med student so I definitely am not a doctor, but on my way to become one. Black stool is always a good reason to call your doc. Usually it's a sign of some sort of bleeding in your GI tract (i.e. stomach, intestines). It could possibly be some sort of ulcer as that can lead to some blood in your stool. You also usually get abdominal pain kind of above the belly button when you have an ulcer. A lot of times this is caused by a bacteria, which they can medically treat you for with antibiotics. Do you take a lot of ibuprofen, aspirin, or caffeine? Those are other causes. It sounds like you may have an ulcer based on the fact that you have the black stools and the abdominal pain. Obviously there are other causes, so it's good to see your doc. They'll probably want to check your stool for blood. Anyway, you really shouldn't be embarassed by it....most people in the medical field are VERY IMMUNE to anything with pooping or peeing and aren't even phased by it, so no worries. Anyway, my point is you didn't overreact by calling the doc

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Edit: I agree, it could also be gallstones (even though you're pretty young) as some of the risk factors are being female, at an age where you can reproduce, and also overweight (which you are working on!). Usually you will have pain after eating fatty foods....what kind of food were you eating when you had the pain? Was it more all over the abdomen or just in one area?
I'm sorry you're worried and not feeling well