Inventory keeps flipping back and forth between two kinds.

Okay, so say I'm playing on an offline server with my brother. I play for a while, get a bunch of stuff, then log on. Next time I log on, my stuff is gone. EVERYTHING- Inventory, hotbar, armor... everything. Now say I continue to play and get more things. The next time I log on, Inventory #1 is BACK. Now this doesn't happen all the time, but Inventory #1 and Inventory #2 keep switching back and forth at random. It's really frustrating, b/c all my good stuff would be in Inventory #1, but then I would switch back to it after I got a bunch of good stuff in Inventory #2. Please respond ASAP so I can get back to playing normally.