When a
bedikah cloth with questionable stains is lost (or discarded, laundered, etc.), we must assume the stains on it were problematic, since there is now no way to check if they were acceptable. Unfortunately, unless you are certain that the stains on the morning
bedikah were acceptable, you must assume your
clean days were invalidated. Your afternoon
bedikah from Monday counts as your new
hefsek taharah and your immersion is postponed one day, to next Monday night.
Please feel free to get back to us with any further questions.