I have a 17 month old with similar issues. A non stop runny nose which is what is causing the cough. Our pediatrician believes he has mild dust allergies (very common). She gave me some info on it and we have been following it. It helps a great deal.
-no carpeting or drapes in his room. We have hardwood and blinds on the windows.
-the cat is never ever in his bedroom
-every other day I not only dust his room thoroughly and wash his floor, but I also sponge down the walls, blinds, etc.
-no stuffed animals in his room
-I wash his bedsheet and blanket every other day
He is seeing an allergist in august, but she really thinks this is what is going on. As long as there are no signs or symptoms of asthma (which there have not been), she isn't overly concerned. Following the list I gave you has really made a difference. We also have an air purifier in the house now, and I pretty much restrict his play area to upstairs which is hardwood vs. downstairs which is carpeted. He still has the runny nose and therefore cough, but it is way better. Our pediatrician feels he will likely just outgrow it. The mucus can also irritate the stomache and cause vomiting, although we have only had that happen on occasion.
Obviously I don't know if this is what is going on with your child, but perhaps it will help?
Yes it may help , in many children milk adds to congestion especially at night , as your child is 17 months old it could be less milk would be better as there could be some intolerance going on , how are her bowels ?
Yes i have a humidifier for her it helps a bit. She does drink milk before bed thats an interesting comment maybe i should try her with water instead?
Does she drink a lot of milk as that may not be helping, milk can create congestion in some children .?
Have you tried a humidifier in her room at night?