Spawn chunks regenerated

I actualized minecraft from 1.14.4 to 1.15 and I opened my world normally, then I noticed it was too lagged so I went back to version 1.14.4 where I had optifine so it was supposed to be faster but when I entered my spawn chunks where I had all my constructions and stuff vanished and appeared like they were when I first created the world, but I kept all my stuff, then I got into creative world to look around if I was in a different place and I notice that everything that was far away from the spawn chunks was normal. Please if there is a solution contact me.