Hello Mrs., thank you for your question!
Decongestants can decrease milk production, so they should be avoided while breastfeeding.
For throat irritation, acetaminophen (Tylenol and other brands) is the best treatment, as well as lozenges like Fisherman's Friend for local relief.
For coughs, mucus discharge, and nasal congestion, I would opt for nasal hygiene with a saline solution (Hydrasense, Sinus Rinse and others), which can be done as often as needed during the day, and at least 30 minutes before bedtime. By 'cleaning' the nasal secretions, you can reduce the backward flow of secretions, which often causes coughing.
I also encourage you to drink plenty of fluids, cold or hot, as you prefer.
I hope I've been helpful, and please don't hesitate if you have any further questions!
Tom Samaha, pharmacist