Fb unfollow feature and activity log- some discrepancies

While browsing through my activity log on FB ,I came across some weird notifications. My activity log says that I unfollowed one of my Fb friends on 20th February, 2014, when in fact I never did so. Upon visiting her profile, I saw that I had indeed unfollowed her and she had kept most of her pics and all of her status updates locked from me. Only her public posts were visible to me on her timeline. But I distinctly remember that I had never unfollowed her on my own.
Please help me out with this by stating the possible reasons why this could have happened.

Did you get any notifications from Facebook that there was an attempt to log in with your account from an unusual place? Or maybe somebody has an access to your account or device.

Because your memory is playing a trick on you?

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