These slight oscillations always happen until everything is open.
They are due to one of two causes,—either
(1) Some small part or movement of the being comes up which is not quite open and needs to have the Influence brought into it, or
(2) A shadow is thrown by the outside force, bringing back, not the old disturbance, but some temporary obscuration or appearance of obscuration.
Do not be disturbed, but immediately become quite quiet and open yourself.
The important thing is not to allow the old strong disturbance and confusion to come back and, secondly, not to allow a long obscuration, even if the obscuration be without a serious disturbance. To keep hold on quiet persistently will prevent the serious disturbance; to keep quiet and steadily open yourself will prevent any long obscuration.
Ref: Letters on Yoga – III