Why did my MineCraft FPS drop so much? - 1

Ok, so I have an HP Envy desktop PC. On MineCraft 1.6.4, I would get 300 FPS on max settings. So one day I came home from school, got on MineCraft, and My FPS dropped down to below 60 FPS, I turned all the setting down to the lowest possible and it still remains below 60 FPS with huge, constant lag spikes. I even allocated 4 gbs of ram to the game and it still hasn't fixed anything! Someone please tell me why this has happened!