Swallowed bone. It depends on the size of what you swallowed. If it was a round-ish or small piece and does not feel like it got stuck anywhere on the way down, it will generally pass by itself. If it is larger or sharp and/or feels like it got stuck on the way down, you should go to the er and have it retrieved to prevent it from possibly perforating the digestive tract.
Maybe nothing but... Small bones may actually pass thru the gut. However, large or elongated structures have the risk of turning sideways and getting stuck at any point in the GI tract. Bones can be endoscopically retrieved when they've made it no further than the stomach. The ideal time to get help then is within 4 hours of ingesting the bone. If after that & you have obstruction symptoms, go straight to the er.