There's no "perhaps" about it. Sr. Faustina Kowalska experienced a lengthy series of visions and personal revelations, which she recorded in a diary. After no little controversy within the Church, after her death (IIRC), her diary was published and led to the somewhat popular devotion to "The Divine Mercy" ... my initial post linked a site promoting that devotion.
Lest there be any confusion, we (Catholics) do not regard personal revelations to be in any way equal to the Sacred Scriptures ... belief in or acceptance of even "approved" visions or revelations is in no way required.
There has to be someone not telling the truth here,either sister or God,I think I will lean toward believing Gods word.. :)