You might have what is sometimes called "fibrocystic breast disease." One good doctor told me that he thought it was pretty dumb to call it a disease, since more than 50% of women develop it at some point in life. I agree with him.

You can kind of tell by checking if you have lumpiness. It may come and go. An OB-GYN can tell you, if you are "cystic," and show you how to feel for that.

Since the heaviness went away, it's unlikely to be anything to worry about. See if it recurs and if there is a pattern to when you get it.