There can be staining after Mirena removal.  One type of staining is a direct result of the physical removal of the IUS, for example if it scratched the vaginal wall on exit.  This type of staining is considered dam makkah, blood from a wound, and does not make a woman niddah.  Typically, this type of staining shows up within a day of removal. As your physician said, this spotting can last for a few days. The bleeding can be assumed to be dam makkah for the first two days after removal, unless it is the time that you expect your next period (a veset day, which in your case is not relevant).

A second type of staining after Mirena removal results from the body’s adjustment to a new hormonal environment, without the progesterone secreted by the IUS.  This type of staining is uterine in origin and can make a woman niddah if it meets the halachic criteria for stains. We suggest reviewing our article on stains, which may become relevant as your body continues to adjust to the absence of the IUS.

You can and should continue to wear a pantiliner, as stains on a pantiliner will not make you niddah. A flow of blood, as opposed to light staining, would make you niddah. It can be difficult to discern exactly when staining becomes a flow; bleeding heavy enough to require a pad is certainly a flow.