I took 10-days worth of progesterone after my mock cycle ended and STILL didn't start bleeding for another 3 days after stopping the progesterone. If you consider the body's natural cycle, though, that's about right... The lining should be very thick around the time you'd naturally ovulate (when estrogen is high, so when you had your ultrasound to evaluate the lining) and then once estrogen drops off, you get a progesterone surge, which is what causes the lining to shed about 10 days later.
I suppose if your body is producing enough progesterone on it's own, you will eventually start bleeding. However, since my body does absolutely nothing correctly and certainly doesn't produce any hormones, the progesterone was necessary with me.
I'd call. But FWIW, I took 10mg of Provera for 10 days.