Question:

Why does my throat feel clogged up when I sing?

Answer:

There are many causes for congestion. If your voice normally feels clear but causes more phlegm when you sing, it could be that the resonance (vibration) is loosening congestion and causing a nasal drip and the need to clear your throat. Allergies and acid reflux can also cause phlegm. Your best bet always is to see a doctor.