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The increase in breast size and soreness can happen due to hormonal changes also. Has she been regular on her pills? If yes then there is no need to worry about pregnancy. Wait till she finishes her entire pack, she should get her periods (periods can be delayed normally upto 5-10 days).
If she develops any other symptoms, ask her to report to her healthcare provider/gynecologist at the earliest and get a thorough examination and investigations done to make sure that it is not due to any other serious cause or pregnancy.
Take care.