J.M.
Hi! Yes, do call the pediatrician's office! You're on the right track. If they don't take you seriously consider a second opinion. There is a condition called Encopresis. Sometimes when a child is constipated, they begin to hold it to avoid voiding. This in turn sets up a situation in which they begin to have difficulty controlling their ability to void. It is a physical problem that often leads to huge power struggles as Mary Sheedy Kurcinka writes in "Kids, Parents and Power Struggles". She also mentions in the epilogue a resource for dealing with this issue, "Childhood Constipation and Soiling" by Dr. Judith Owens-Stively.
I totally agree with the other responses I had a chance to quickly read. A friendly attitude is a must in power struggles. You don't want to get hooked in because, darn it, kids are smarter and more persistent than us and they do win every time. :) Even if winning looks like us getting exasperated. I take a lot of deep breaths and need to take especially good care of myself when working through an issue so I have the patience to not let my buttons get pushed. I've been working on power issues with my four year old and since I've been working really hard at maintaining a friendly attitude, side-stepping power issues, and giving him lots of choices and teaching him self-sufficiency, It's like a have new, delightful kid!
It took a few weeks to turn the boat around, but it was worth the effort and things are easier as my new attitude has become second nature too. Also, when you start to work to improve misbehavior, sometimes it gets a worse before it gets better as they pull out all the stops. Just know that if that happens, it means the change for good is just around the corner and hang in there. Best wishes!