What Could Be Causing Frequent Colds, Often Triggered By Minor Irritants?

Hi .. I'm 25 yo female ... I get cold very frequently . Thrice a month recently . I get cold. I complete my medicine course. Get better in a week or so ...after some days i catch it again.... sometimes it takes a minute particle of dust .. sometimes tomatoes ( a single serving spoon that too once in 3-4 months)... Is enough for inviting my cold....
I really get very irritated by my cold .. it feels like I'm sick everyday ... Please tell me wht to do ... N wht to eat ... This tym also I'm having cold .. but i had sore throat first (possibly because if ice cream) then it developed into cold ... .. for which im on medicine (antibiotics + cold medicine (otc). I almost completed my course... But it didn't give me much relief..