Thank you for helping.
No, I haven't upgraded to MM nor did I install any app prior to this.
"Power button instantly locks" is on and automatically locks is set to 15 seconds. I haven't touched the "smart lock" feature except when I first got this phone several months ago.
I am noticing more problems with the phone now. One is that it takes too long for it to charge now. I've plugged it for 5 hours and the battery only charged up to 66%. Another is sometimes it doesn't respond to touches. Also it frequently freezes now.
Thank you for responding!