Did you simply drop your items on the floor? Stuff in the ground piles vanishes after some time, this is how it's supposed to work. If you need to dump your inventory somewhere, find any actual container and put it there. Personally, I start out using the boxes in that small rat-infested house in Ragpicker's square; later on, you'll have access to more storage space, so you can move your stuff wherever you find it convenient.