Thank you for reaching out to us.
Even a very small amount of red can invalidate a bedikah or hefsek taharah. In cases like these, we usually recommend having the cloth evaluated. If you’d like to share your location with us, we can try to make a referral to a local Yoetzet Halacha or Rabbi who could do this.
When a woman attempts a hefsek taharah and is unsure of its validity and it is not yet shekiah (halachic sunset), she can consider making another attempt prior to shekiah of that day–even cleaning herself a bit beforehand–just in case the initial attempt was not valid.
We hope this isn’t too late. For time-sensitive questions such as these, we recommend contacting our telephone hotline, details here.
Please let us know if we can be of further assistance.