What Causes White Pieces From Throat During Coughing?

hi, sometimes i have little white pieces that come up my throat. i can feel them in the back of my throat but i have to force myself to cough for awhile so that i can get them into my mouth so i can spit them out. also when im running, all of the hard breathing helps to force the pieces out as well. i also noticed one i spit them out they smell like very stong feces odor to them