WHAT IS THIS ???? (Undertale)

I went through the game killing everything that moves but then at the end of the fight with the Sens wasn't and I went on killed Azriel of Toril appeared appeared Flavi and said thank you that I have oslobodi all the souls but then the game crashed
I rebooted it and then the monster on the initial splash screen has changed, the game has become dramatically change its name until it disappeared altogether then came the preservation flowey lv9999, I was scared
And then the muzzle flavi begins to broadcast about the fact that he needs another shower
I'm in a panic here comes the mutant insect and flavi on top of nachilsya bossfight the standard menu was not just flying soul when there was a attack button I pressed caused 1 damage flavi or who was there killed me then I relaxed because there was the usual game over they say everything is a nightmare HERE AND YOU'll NEVER WAKE HAHAHAHAHAHAHAHAHAH the game crashes again and again and again and all over again I'm afraid help is on creepypasta it seems not undertail