What's the probability that these are early appendicitis symptoms?

Sarah, at the time you first posted your Question on Answers it would have been too early to know specifically what caused your umbilical area pain. IF it was an early appendicitis, it would likely have progressed within hours as the pain moved to the lower right abdomen. Rebound pain often develops when you then gently press down on the belly and suddenly let go. Pain would greatly increase and fever might develop. I hope you're over the problem though it's good that your consider such a possibility.