Although it is preferable to perform the hefsek taharah before minchah ketanah (within two and a half proportional hours before sunset), a bedikah performed earlier in the afternoon is valid. This timing would not change the psak you received about the stained cloth.
In fact, we generally recommend that a woman should perform a backup hefsek earlier in the day, so that if she later forgets or misses sunset, she may rely on her earlier hefsek.
(Note: If a woman performs a hefsek taharah on the same day on which a new flow began, as may be the case during the shivah neki'im, the hefsek taharah must be done after minchah ketanah. In all other cases, an earlier hefsek is valid.)
Please feel free to get back to us with any further questions.
B'hatzlacha!