A speculum exam may cause bleeding from the vaginal walls. Such bleeding does not make a woman niddah. Since the bleeding you report is consistent with bleeding from a speculum exam, you may attribute it to the exam and you are not considered to be in niddah.
(It is not entirely clear from your question whether the stains you saw would have been sufficient to render you niddah in any case. A stain on underwear will make you niddah only if it is larger than a gris – about the size of a US dime or an Israeli shekel – and the underwear is white. The status of stains on toilet paper is subject to halachic debate, and may depend on the particular circumstances. For more information on these laws, please see our articles on Stains and Toilet Paper.)
If your physician saw blood leaving the cervix, if you are expecting your period, if you had any heavier bleeding, or if bleeding persists for a few days after the exam, please get back to us or a halachic authority with more details.