My son would do this PARTICULARLY at night (the post nasal drip would trigger his gag reflex). We gave him pseudoephedrine (aka, the Sudafed behind the counter that you have to sign for these days). The antihistamine stuff they've replaced it with is totally ineffective for most people (including our kiddo).
The pseudoephedrine was/is an absolute godsend for us. The vomitting is not only exhausting for him, & messy for us, but the danger of him aspirating on it in his sleep... Icewater for blood. Easy enough to dose correctly by weight (not age).
I personally avoid most "natural" medicines. AKA sold as natural/homeopathic remedies. Many aren't tested, or regulated, the doses vary wildly between both brands and BATCHES, and there's lousy science/research/longterm studies on many to most of them.
I say this as a person who LOVES herbal pharmacology (and my cousin IS a pharmacist who is currently researching medicinal properties of ancient religious/medical treatments. for her 2nd phD... fascinating stuff) BUT the *industry* is absolutely shameless. Hylands in particular gets me on my soapbox, and they're one of the "better" ones. Take the same ratio of teething tablets for YOUR body weight and you'll be drooling and giggling on the couch/floor high as a kite, with some really entertaining hallucinations (highlands has nerve toxins, hallucinogens, and euphorics in it). Oy.
Whoops. Soap box :) Sorry. The mucus needs a decongestant, the cough needs an expectorant. The best OTC decongestant available is pseudoephedrine (there are stronger Rx decongestants, but they aren't usually necessary unless your doc is treating pneumonia). The best expectorant is Guaifenesin. You don't want to use a cough suppressant right now, because the cough it productive. Get that phlegm out of there :) As always... push fluids, and keep her on a high fat and protein diet (antibodies are made almost entirely from fats)... Chicken soup is the perfect kind of high fat/protein.
((Gauifenesin comes from the Guaifa tree, Dextromethorphan -a cough suppressant- is essentially the inner bark of a certain kind of cherry tree, Ephedra -aka pseudoephedrine- occurs naturally in MANY plant species including Ma Huang (never mistake chinese names... a chemical is a chemical no matter what language it's spoken in). MOST of our medicines -including opiates- come from plants. Just because something is natural (rattlesnake venom is natural) doesn't mean it's safe.))