Sometime a drop of blood came out after urinating what is the reason behind it?

Urethrorrhagia. Not uncommon in boys around & just after puberty. May have a little blood spotting on underwear. Usually no other symptoms although some have mild burning on urination. Worth urinalysis to rule out UTI, may last year or 2. Can be associated with urethral stricture in which case urinary stream will be slow & weak. Needs 2B evaluated only if stricture is suspected. Otherwise no work up.