Get to the vet right away. One of my cats did this, and I found out he had swallowed a metallic christmas ribbon. The doctor had to cut it out (it was 3' long! A friend who gave me the gift wrapped in the ribbon blames her mother, who gave her the ribbon. I blame me, because I had been playing with him with the ribbon, and never imagined he'd find it himself and swallow it!)
My vet said that when animals swallow some things like this, it can ball up and not be able to be passed through the system, so it stays in one place and irritates that place, and can literally shred the intestines to a point where they are irreparable.
Don't mean to scare you unnecessarily, but to emphasize the importance of gettin to the vet right away. It may be nothing serious (let's hope not), but it may be something that is time sensitive to deal with.
Let us know what happens.