Besha'ah tovah!
It sounds as though you may have had implantation bleeding, which is uterine. Uterine bleeding can make a woman niddah, even when she is pregnant. Should you determine that you are now in niddah, you should perform a hefsek taharah and complete the usual process of becoming tehorah.
Whether the brownish reddish stain you saw on toilet paper made you niddah depends on a few factors, including the time elapsed between urinating and wiping, whether you follow Ashkenazi or Sefardi halachic rulings, the exact color of the stain, and the lighting conditions under which it was seen. For more information, please see our article, "Toilet Paper".
The spot on the tampon may itself have made you niddah, depending on its exact color. A light brown with no hint of a reddish tint does not make a woman niddah. A darker or reddish shade of brown may make one niddah, and should be evaluated by a halachic authority. Accordingly, if the toilet paper did not make you niddah and the color on the tampon is in question, please bring it to be evaluated. If you no longer have the tampon, please write back with a clearer description of the shade you saw and the lighting conditions under which you saw it.
Since spots on tampons are treated more stringently than those found externally, we usually advise women not to insert tampons until their niddah status is certain.
Please write back with any further questions.