One sign of labour beginning is a sudden drop in temperature, so that may be one of the reasons she is shivering. Of course, pains cause dogs
to shiver too.
are pregnant for about 63 days, though obviously it can vary a little, a day or so each side of that. So if that much time has passed since she mated, I think you can probably say it is her time to give birth.
Usually the birth will go without complications, but do be prepared for any problems, because they CAN happen.