Why cant i stop coughing?

I am just about getting over a pretty bad cold bordering on a flu and all my symptoms like chills, weakness, runny nose and sneezing have stopped pretty much but i cant stop coughing and when i cough there's like mucus or phlegm that comes up, its not like one cough and thats it once it starts i literally strain myself coughing, i also have asthma if maybe that has something to do with it.