You don't say what your babysitter does to get him to nap.
Baby can pull up to a standing position from a seated one. If he cried I would first try to comfort him in his crib. Baby shows unhappiness when you take something away. If you haven't already done so, now would be a good time to move him into a full size crib, in his own room if possible.
These neat little things are great for soothing a baby, a baby putting itself to sleep, and of course. It does sound like her post-waking behavior kind of matches this. You'll know the end of the cycle is in sight when the kid starts waking up later and later, even if it's only by 15 minutes every night or two.