One possibility is fluid retention, I would guess. On VLCDs people drink tons more fluid than usual and it has to go somewhere!
Some do have breast tenderness around T.O.M. Also - during the early stages of pregnancy. I sure did! If there is any chance at all that you might be pregnant, have a test done asap. Either way I would see the doc, as a precaution. One can never be too careful when it comes to health.