What Causes Blood In Phlegm?

Hello doc, I do not smoke or touch any tobacco products. I have a history of sinusitis infection and during my last infection a week ago I had noticed blood in the phlegm which I though was due to the infection. After couple of days using antibiotics the infection has subsided but the blood has not stopped coming from the initial cough in the morning and that too very little of blood. This is little bit worrying me since I had not seen this before. I had done the TB (AFB) test which was negative. Also the chest x-ray is clear showing no signs of issue in the lungs. I do have a dizzy head with a mild head-ache. Can you please help me out here? Thank you.