What can cause you to cough phlegm with a little bit of blood?

Commonly, infection: Infection such as bronchitis, is the most common cause of blood tinged sputum. However, if this persist , other conditions such as tuberculosis, abscess, cancer, abnormal blood vessels, blood clots, fungal disease may be present. If it doesn't stop on its own, seek medical attention.

Hemoptysis: Bronchitis/pneumonia/congestive heart failure/pulmonary embolus/ ling cancer with airway involvement to name a few. If symptoms persist, you should seek medical attention sooner rather than later.