sore breasts have other causes. I know someone who was sore with fibrocystic breast disease and found that eliminating, all dairy and animal products made it less frequent. Sometimes if the man gets a bit too happy there they can also get sore.
Other diseases or infections can make them sore and breasts should be self examined every month they say about a few days to one week after periods.
A health professional could of course help to figure out the cause.