Is it bad that one of my breasts keeps growing and shrinking? What does that mean?

It's starting to get a little concerning and sadly I don't have the funds to go to the doctor to check it out unless I absolutely have to. My right breast will get bigger and then stay bigger for about a week, and then go back to just about the same size as my left and when I do a self check it doesn't seem to be too fishy but I have no idea, I'm not a doctor D: