B’sha’ah tovah!
Yes, that does make you niddah. Any amount of blood found on something inserted internally, like a tampon, can make a woman niddah. The leniency for stains smaller than a gris applies only to stains found externally (learn more here).
For the future, if you want to use protection just in case your period comes, we recommend using a disposable pad. Since the pad is external, all the leniencies of stains would apply, and a small quantity of bleeding would not make you niddah.
Please feel free to get back to us with any further questions.