Often times my ribs feel like they are convulsing or shifting. what could this be? sometimes it hurts but sometimes it doesn't. no previous injury

They can move: Your ribs all move. that is how you breath. Some are attached at both ends breast bone and back bone. The others only on the back with the front floating. They all move to breath because muscles pull them up. You are probably having muscle spasms (cramps) It will likely go away after a few weeks. If not see your physician.