Can a person be betrayed while asleep without feeling it, and what could be the reason for the presence of blood in the underwear after waking up without feeling any pain?

A Muslim must think well of other Muslims unless there is evidence to the contrary. Allah Almighty has forbidden ill thoughts, saying: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." The presence of blood could be due to a simple reason like a cut or a boil, and there is no need for such confusion.