When a woman declares herself niddah based on misinformation or an error, she may retract once she discovers the error and clarifies the situation.
Therefore, you may consider yourself tehorah since you mistakenly believed you were niddah from the spotting following the IUD replacement. You should continue taking precautions against becoming niddah from any further staining by wearing colored underwear and waiting a few seconds (ideally 15 seconds) after urinating before wiping.
Please feel free to get back to us with any further questions.
B’Hatzlacha!
This response has been updated to reflect the rulings of our current Rabbinic Supervisor, Rav Kenneth Auman, regarding waiting before wiping.