Is this an allergy possibly?

I've been having problems for about the last month. It started off as a normal cold which became a sinus infection. All the symptoms of the cold went away with the exception of the post nasal drip, coughing, and the periodic nasal congestion. I'm still on antibiotics for the infection but they don't seem to be doing any good anymore. At first they did. The thing is. I can almost set my watch by the times of day that I start coughing. I also seem to cough a lot less while sitting or lying down. It seems like I start coughing at 9:00AM and stop coughing, at least a lot, around 6:00PM. This is pretty much regardless of where I am. If it's an infection then why is it only hitting at certain times of the day. If it's an allergy then why doesn't my location make a difference instead of time of day? I can't figure this one out.