Today I was playing in my sisters Minecraft world and I accidentally went onto another window which made me leave and I had to join back in but when I joined back in I lost all my stuff and spawned not at my bed, but I spawned at the world spawn that when you first join in you spawn at. It could just be my bad ram computer though.  ps. but then I accidentally went onto another window again and when I joined in I got all the stuff I had back and I was in the same place I was in before! odd but you should still probably fix this problem