I can think of two possibilities:
(1) hormonal fluctuations. This could be anything from the monthly, which affects chest shape and size in a lot of people, to puberty changes (I don't know how old you are but if you're under 25ish puberty is still a possibility), to something external like starting a new birth control or whatever.
(2) swelling from injury. If you're binding tightly enough to damage your tissue, then it wouldn't be unusual for it to exhibit normal signs of injury, one of which is swelling.