Sinus problem?????

I have had a tickly cough for the past couple of months and a feeling of weakness in my chest. I don't have the cough durng the night-it starts after I get up and seems to be as a result of mucus going down the back of my throat. Every morning I have catarrh in the back of my throat when I first get up-this happens all year round. There was a day last week when it rained all day and I didn't cough once! (I live in the country so there may be lots of pollen but they don't seem to think I have an allergy cos I'm not wheezing) My doc prescribed a 5 day course of antibiotics and a Ventolin inhaler-neither helped. He has sounded my chest twice and saysit sounds clear.I saw him the other day about something else and mentioned things hadn't cleared. I sawe a student doc first who took lots of notes and he suggested that it may be a sinus problem cos it can cause a nasal drip (my nose isn't runny at all!) and said a decongestant might help. When he gave my doc a synopsis of my symptoms my doc prescribed a 7 day course of the steroid Prednisilone. I am reluctant to take this as I have Multiple Sclerosis and have had them before when I have had exacerbations and I'm not keen on the side affects!! Does anyone else have anything similar to my symptoms? This is driving me mad and I'd love to know the cause as I'm beginning to worry about it.[:@]Any suggestions on medication?
Thanks, Galadriel