Should I keep taking the Robitussin or see my doctor?

I'm 37 weeks pregnant and have had a bad cough. I cough up some phlegm/sinus drainage in the morning, but the rest of the day it's mostly a really hard dry cough. I do have asthma, but during my pregnancy it's never been this bad. I have taken 4 doses of the Robitussin throughout the night so I could get a little relief. It seems as if the cough is worse this morning. I have an ob appointment tomorrow morning and have tried to get a hold of someone in the office with no luck. Should I continue to take the Robitussin and wait for my appointment or get in to see someone today?