You may want to have the vet check her teeth. It's odd that she would start this behavior after 5 years unless you 1) changed your schedule (started a new job that keeps you away more, different hours, etc.), or 2)introduced something or someone new into the home (new baby, new boyfriend, new puppy/cat, etc.)
If she has a sore tooth, she may be chewing to numb the pain. With some types of injuries, the pain is lessened when pressure is applied. She may be doing this. Best to get it checked out before you write it off as a behavior problem.
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