Thank you for reaching out to us.
You certainly can count your bedikah on Wednesday as a hefsek taharah.
Whether the tampon could count is a good–and complex–question.
A hefsek taharah usually entails manipulating the bedikah cloth with one’s finger so that it reaches all vaginal folds and crevices. Though the tampon fills the vagina, it does not necessarily get to these. Additionally, because of a tampon’s absorbent nature, it’s hard to know or evaluate what has been absorbed within it. In your case, there is the further complication of your not having intended to use the tampon in this way. Your description of it as lightly soiled also raises questions.
For these reasons, we do not usually rely on a tampon as a hefsek taharah. The exception is when there are specific pressing circumstances (e.g., marital or fertility concerns). If there are special considerations in your case, we might be able to rely on the tampon, depending on details of the color of the soiling that you saw (ideally observed in daylight in the shade).
Please don’t hesitate to follow up with more details.