hai, the most probable and least serious cause can be a throat irritation after recent throat infection which has cleared as you said you had green / yellow sputum. for this keep drinking warm fluids, take cough suppressant like bronchophane (sedative, controlled medicine so limit the use to minimum say 5ml 3 times a day or even less) and chewable lozenges like strepsils. But it can be other causes like acidity/ gastritis also. so make sure you sleep at least 2hrs after your dinner. take antacid like gelusil mps 10ml 3 times 30min before food. Try these for 1 week, if no improvement consult.