A childhood belief about under my bed

Like many young children, I enjoyed jumping on my bed, espcialy since I was not suppose to do it. One particular time I was jumping, I looked at the opposite end of my bed and saw a "bump" go up when my feet touched my bed. I was a little shocked, so I did it again to see if it would happen again. It did, and I immediatly thought that it was some sort of monster on in my matress that got scared when I jumped on my bed. I even tried to jump on the "monster", but was a little angry when I saw that it moved to the other side of the bed. It was a few years later that I realized it was just my bed going up due to my jumping.