I think this is more probable if you missed a pill, and had to make it up. This has happened to me a couple of times. But I tend to miss, and then have to make it up. (A dead give-away that I am single right now?
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Stress, and other things can cause this too. Unless you are in extraordinary pain or are bleeding more than normal I wouldn't worry about it.
You could always call and talk to a nurse to make sure though.