What Causes Discharge Of Green Phlegm With Foul Odour From The Mouth?

Hi: Sometimes in the mornings I get a small particle (not mucusy but has the texture of a sponge like substance). It is very very small and green and has a foul smell that usually fills my mouth and make me gag. I think this comes from my nose but I am not sure why? I am healthy and have no infection symptoms. Any suggestions?