I've had a cold for six days. Why does my cough get worst at night and when I lie down? I've been taking Robitussin (guaifenesin) and sudafed.

Post Nasal Discharge. Try saline nasal rinses to thin and increase the flow of mucus. As your cold improves, the post nasal discharge and cold will resolve. It helps to stay hydrated as well.
Post nasal drip. When you lay down, snot from your nose drips down and collects in a pool. Your body coughs it up so it doesn't go into your lungs.