God only knows for sure. I think it all started with an inflamation of the duoden (for some unknown reason), which gave me some serious cramps for a few weeks. (I did not go to the doctor though.)
Then they vanished and for almost a week I was perfectly fine. Suddenly - without any pain to speak of -perhaps because of the stress of learning how to drive - I started to feel bad. It was Friday. I felt no pain in the stomach these days, just some weird nausea and I was getting tired quite fast. Saturday I went with my brother downtown. I had to stop walking a few times. I was out of breath. By Sunday I was white as a sheet but conscious and otherwise in good spirits.
On Sunday, although (strangely) I was feeling slighly better despite my lack of coloration, a neighbor, my best friend's mother (she's a nurse), hearing about my condition, urged my parents to send me to the doctor.
The doctor was stunned. He had no explanation at that point. They run some blood tests and it later turned out that the red blood cell numbers were almost half of what they should've been.
After the rectal test one of the doctors told me "Melena! Lie down and don't get up." A week in the hospital followed. (A very short time as I recouped quite fast.)
Final conclusions: no one knows why it happened. Perhaps it was because I didn't ate breakfasts at the time, perhaps because I didn't get enough sleep, perhaps a temporary hyperacidity in the stomach, perhaps stress, perhaps something else or maybe all of those combined.
Fact is now I eat normally, drink normally, sleep weirdly (but enough hours).
I live a normal life and I thank God for that. I consider myself a very lucky individual.
PS. Guys, sleep enough hours. Dreams are nice!