from what I've found, marshmallow root is safe during bfeeding.
Eva, if I were you, I would be using natural and/or external remedies for coughs/colds - so lots of garlic, lemon, honey etc, steam your room (with vicks if you can get it), rub vicks or amol on your back, use "banki" (suction cups) if you believe in them, hot showers for congestion (also good for milk supply if you add in massage to your breasts), saline rinses for your nose and throat etc.
I am the same with decongestants - they make me feel as though I've had 10 coffees!
huge hugs and hope its better this morning xxx