2,110. A Stain That's Not as Big as Three
Hilchos Issurei Biah 9:13
Let’s say that a woman found one stain that isn’t as big as three stains combined. If she checked herself during twilight of the first day and saw that she was clean but she didn’t check her clothes and on the third day she found this stain that isn’t the size of three stains, she needn’t be concerned that she might be a zavah. If she didn’t check herself during twilight, then she must consider the possibility that she might be a zavah since she didn’t check her garment and wore it for three days during her zivah time.
Hilchos Issurei Biah 9:14
If a woman found a stain on her garment on one day and then bled for two consecutive days, or if she bled for two days and found a stain on the third day, there is doubt as to whether or not she’s a zavah.