A red stain of any size (even a very small spot) found on an internal bedikah can invalidate the clean days.

If you still have the stained bedikah cloth, you should bring it to a halachic authority for evaluation.

If you no longer have the bedikah cloth, you should assume it invalidated the clean days. In that case, your next valid bedikah counts as a new hefsek taharah, and the bedikah the day after counts as your new day 1. For example, if the stained bedikah was found on Sunday morning, your Sunday afternoon bedikah counts as a new hefsek taharah, your Monday bedikah is your new day 1, and you will go to mikveh on Sunday night. Alternatively, if the stained bedikah was found on Sunday afternoon, and the next clean bedikah was on Monday morning, your Monday bedikah counts as the new hefsek taharah, your Tuesday bedikah is your new day 1, and you will go to mikveh on Monday night.

Please feel free to get back to us with any further questions.

B'hatzlacha!