The estrogen level often dips at ovulation, which weakens the uterine lining and can lead to staining such as you describe. The possibility of hormonal treatment for such staining can be discussed with a physician.

However, not all staining makes a woman niddah or invalidates her clean days. Staining found on a white undergarment during the clean days only invalidates them if it had the area of a gris (roughly, the area of an Israeli shekel or American dime). Smaller stains on undergarments are each evaluated separately for size. Larger stains can be evaluated by a halachic authority for color, as not all shades invalidate the clean days. Stains of any size on a bedikah cloth can invalidate your clean days; if the color is not clearly red they can be brought to a halachic authority for evaluation. For guidelines regarding blood seen on toilet paper, please see our article on that topic.

For the next few cycles, if you see you are staining during the clean days, you may omit bedikot until the next day of the clean days. (For example, you can skip the bedikot of day five and resume bedikot on day six. If necessary, you can even omit the bedikot of day seven and do bedikot on day eight – treating day eight as a new day seven and going to mikveh one night later than originally planned – so long as you have been performing regular bedikot to that point.)

If this step and the above information do not suffice to resolve the issue, please write back with additional information.