Niddah 5 - Fixed Period
If a woman has a fixed period (either her period arrives on the same day of the lunar month, or after a fixed number of days from the previous one), she is not given retroactive impurity: if she finds blood internally at the expected time, all the ritually pure foods she handled prior to this remain pure, and only foods she handles from now on are ritually impure.
If a woman checks herself for blood before and after marital relations, then this check also counts to prevent retroactive food impurity. If the teacher did not tells us this rule, we could have thought that before relations she is rushing "to her house," that is, to be with her husband, and won't inspect properly. After relations, we could have thought that a small drop of blood the size of a mustard seed may be covered by semen. Therefore, the teacher had to tell us both cases.
Art: Pierre Auguste Renoir - Boating Couple Aka Aline Charigot And Renoir