Such a reaction is a stress related response. Some possible causes are droughty soil, soggy wet soil (roots lack oxygen and start to die), and serious infestation of spider mites (controlled with strong sprays of insecticidal soap blasted upward from beneath foliage). Take plant outside of course before treating it.
I hope this helps you get to the bottom of this. If the cause is soggy soil you must let it dry out a little more between waterings. Hopefully its will heal up and recover for you.
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