What Causes Blood-tinged Sputum Early Morning?

Hi I am 27 year make ex- smoker. The problem started with chest pain. I underwent chest x Ray and blood test, the result turned out be to normal. After some days, I observed a tingles of blood in early morning. I visited ENT specialist and he prescribed me some antibiotics. The blood seemed to fine. After 2 months, yesterday I once again started observing blood in the sputum early morning. Once again I done the blood test ( CBC and ESR) and once again it turned to be normal. What would be the cause?