This Morning My Husband Was Kneeling Down Cleaning Up Cat

This morning my husband was kneeling down cleaning up cat throwup off our dining room floor. Because of decreased muscle strength due to peripheral polyneuropathy and myeloma he was using a chair to support himself while standing back up. The chair slipped and he fell on his back saying he heard a pop. Because he sees his oncologist tomorrow he wouldn't let me take him to the ER. He has been alternating heat & ice along with taking Tylenol. He seems to be getting around fairly well but now 5hrs later he is saying he is experiencing slight cramping in his low abdomen. Is this something to be concerned with? He also takes 5mg of Coumadin for pulmonary embolism that hospitalized him a year ago.
Thank you.