Youhave a sinus infection
from your symptoms of headache and coughing up phlegm.For the time being you can overcome it with some treatment for the infection.You can use Otrivin
nasal drops 2-3 drops put in the nose tilted well back,twice daily to be followed by steam inhalation..for 5 days.Also an antibiotic like Moxclav 625 twice daily for 7 days,a cetrizine
tab nightly for 10 days and Tab. Zerodol for 3 days twice daily should surely give u much relief till you can see a dentist
later.Avoid head bath for a couple of days.Also cold foods for some time.