B’Sha’ah Tovah!
It is possible that the staining was still caused by the Pap smear.
In any case, stains on a pantyliner may be disregarded and do not make you niddah. Stains on toilet paper would render you niddah only if you are certain that you wiped immediately after urinating. Even in that case, stains within 24 hours of a Pap smear would be attributed to abrasion from the procedure (dam makkah), and would not make you niddah.
For the future, you should take precautions against becoming niddah from further staining by wearing colored underwear or a pantyliner, and waiting at least a few seconds (ideally 15 seconds) after urinating before wiping. You can see our pages on stains and toilet paper for more details.
Please feel free to get back to us with any further questions.
B’Hatzlacha!
This response was updated on 14 February, 2024.