Found this...
http://www.patient.co.uk/showdoc/23068716/
Is it in just the one or both? Both could be hormonal (related to your cycle or pregnancy/milk glands) or caused by exersion or even a cyst or infection.
Not sure how big you are but I know mine can get sore with exercise, if I lie on my front etc... as they're quite big.
Anything out of the ordinary, lumps, pain, thickening, changes to the skin, bleeding or weeping etc... should be checked out and I'm a great believer in getting your breasts regularly checked and shown how to do it properly so will be good for that as well.
I've had two big scares with lumps but turned out to be a harmless fibrous adenoma as many lumps and such are though always better to check so you can put your mind at rest.