Slept with a fan on. is this the cause for my sore throat?

Yes: Usually a ceiling fan will do two things.One is we tend to mouth breath and that drys out throat.Also dust gets on top and ceiling fans throw this down aggravating allergies and the drainage aggravates the throat.Clean them every 2 weeks.

Hard to say: Pain in your throat can be from infection, trauma, or sometime environmental factors. A fan circulating air in a room is not likely to trigger throat pain. Sleeping in a room with dry heat can dry the airway & cause environmental throat pain. Humans respond best to a general humidity of 35-50% which avoids such pain.