coughing, but why only in the evening?

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I have been coughing for the past 6-7 evenings. all is fine during the day. In fact it's so fine during the day, that I almost forget that I cough in the evening. I say it must be gone, but then at 9-10 PM the coughing starts.

if I don't forget about it, I am going to call the doctor tomorrow, but I bet that I get up, shower and go to work and feel just fine and forget about it.

So what causes this strange coughing?