Lower right abdominal pain, 20 weeks pregnant?

i'm 20 weeks pregnant with baby #3 and for the past 5 hours or so i've had increasingly severe pain in my lower right abdomen, close to my hip bone. i've never had pain like this before, it started out feeling like a cramp that you would get from running, and it never went away. i didnt do anything out of the ordinary today ... what could this be? i'm seeing my OB tomorrow for a check up, but should i be concerned before tomorrow afternoon? my husband just laughed and said i probably pulled a muscle or something but could this be appendicitis or something with the baby? i don't have a fever i dont think .... thanks for your advice!