weird graphical glitch

I was playing survival single player, when saw strange flat structures in the air that would move constantly, then I saw zombies with only a head and a leg, then spiders with only 3 legs, also i had to take a screenshot of desktop because it ran out of memory every time I took a screenshot through minecraft , when i pressed f3 it was only using 23% of allocated memory.it is not a texture problem because i switched to another ressource pack and there was the same graphical glitch(as shown in screenshot 15)