Inventory Resetting

This has happened 3 times within the past day, I logon after waking up in the morning or coming home from school and all of a sudden spawn is different, I lost all of my inventory (which had a lot of important stuff this time), and one time the map had even changed.

Why is this happening? The last time it happened an admin gave me a Diamond pick with some enchants since I lost mine and I don't want to keep bugging the admins for items every time this happens.