A light brown stain with no hint of a reddish tint does not make a woman niddah. Based on what you write, your status is unchanged. You may wish to abstain for a day as a precuation against a flow beginning during relations. But that is merely a precaution and not a strict halachic requirement.
Going forward, you should be careful to take precautions against becoming niddah unnecessarily from staining. Please see our article on Stains for details. If you are staying in a hotel with white sheets, you may want to look for something colored that you can place over the sheet.
It is fine to check toilet paper used to wipe externally in order to ascertain whether you are staining – but only if you waited at least a few seconds (ideally 15 seconds) after urinating before wiping. You should avoid doing internal bedikot that aren’t halachically required, since blood found internally is treated much more stringently.
Please write back with any further questions.
This response has been updated to reflect the rulings of our current Rabbinic Supervisor, Rav Kenneth Auman, regarding waiting before wiping.