Hello. Recently, I created a new world. I didn't use any texture packs/mod packs, just vanilla minecraft. I've accidently fell into the lava and I had no way to get out. I gave up. But after staying in the lava for a couple of seconds, I realised, that I am immune to damage. After I got out from the lava, I tested other stuff. I didn't get any damage for staying in the water, lava or fire. And I still didn't get damaged. I am somehow immune from those things.
Any ideas what could it be? My world was created with cheats, so commands might help somehow fix the problem.