Hi, there are many causes for blood in sputum like tuberculosis, heart disease, lung cancer, bronchitis etc. Your X ray is normal which rules out tuberculosis but do you have fever or some other symptoms like cough etc. Sometimes, prolonged cough leads to breach of mucous membrane of lung tract giving rise to blood in sputum, it is self limiting condition.
Please get ECHO heart done to rule out mitral disease etc. Also, sputum routine microscopy and culture should be undertaken. Consult the doctor for evaluation.