I constantly feel like I have phlegm in my throat. What could it be a symptom of?

Sinus/throat/lung/gi. Phlegm in the throat can come from infection in the sinus, throat, lung (bronchitis or pneumonia) and gerd. Each of these conditions can usually be identified and treated sometimes in combination. Sinusitis with antibiotics/decongestants, bronchitis /pneumonia with step wise asthma treatment/antibioitcs and gerd with antacid diet GI medications.
Inflammation . Phlegm can be a sign of post nasal drop caused by allergic rhinitis to environmental irritants or food esp dairy products. It can also be astom of gerd or lpr. The stomach acid irritatesthe esophageal or laryngal mucosal surface causing excess mucus formation.