When a woman omits the bedikah on onah beinonit, she remains tehorah but the couple may not have relations until she makes up the bedikah. So you still need perform a bedikah, even if some time has passed. If it comes out acceptable you will remain tehorah until you next experience a flow or find a problematic stain.
You are only required to calculate vesatot from a flow. This means that if you had performed a bedikah on your onah this month, even if you had become niddah from it due to staining, you would not have been required to calculate any vesatot for next month. Therefore, next month you may take precautions against becoming niddah from any staining (by wearing colored underwear, waiting after urinating before wiping, and abstaining from intercourse while staining). You are not required to perform a bedikah (nor should you). Only if you experience an actual flow, or find a problematic stain (e.g., larger than a k’gris on white clothing or on your skin) would you be rendered niddah.
Please feel free to get back to us with any further questions.
B’Hatzlacha!