cows spawning everywhere

i have been playing on a single player map for a very long time with no online turned on what so ever. i just got done constructing a building when my game became super laggy. i looked around and found a mass of about 500 cows trying to fit in a 10x10 block was there a bug in the system and is there a way to fix it, and please tell me i wont lose my map. i dont know where they came from but after the new update it seems like all the cows just spawned right into my kingdom please help!