What exactly is “breast tenderness”?

Ten days ago after my boyfriend pulled out I took Plan B three hours later just to be safe. Since yesterday I noticed that only when I touch my nipple, it feels a bit sore and sensitive , like they have just been sucked. They also feel slightly heavier but I can’t really tell. Is this what the side effect of “breast tenderness” means on the Plan B box? Or could this be something else? Thanks.