Coughing for way too long

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I have been having this cough that comes and goes during the day for the past 3 months. My allergies just recently started so I know that's not part of it. For awhile I thought it was a mild case of bronchitis. It gets worse at night when I am laying down, so I have to position myself just right so I'll have less of a chance of coughing. Between my coughing and allergies, I don't get much sleep. Has anyone else had a cough that lasted this long, or have an idea what it could be? I don't really cough anything up like you would with bronchitis but I heard some people don't when its a mild case. I have this prescription that's for a cough that has codin to help me sleep which I take at night. I had it when I was sick a long time ago and just kept the rest that I didn't use. My mom said the doctors usually prescribe codin to people with bronchitis and if you don't take it till its gone, then it will eventually come back. Anyone that has had this before or any suggestions, I would truly appreciate it.