Sore swollen breast - hormones playing up?

[tunasandwiches]

On Saturday one of my left breast became swollen and hard – the left hand side of the breast feels full and solid. It came suddenly – wasn’t there in the morning, was by the evening, and has remained sore ever since. Is this likely to be hormones playing up – seems slightly odd that they would only effect one side? If it were you, would you leave it and hope it will go away? or would you see your GP? – and if so, how long would you leave it before going - a few days? a week? a month?
I have a bit of a phobia of doctors and prefer to leave things to pass of their own accord if I can.
Thanks...