It is theoretically possible for there to be some hormonal effects of orgasm that can lead to staining. If this continues, we suggest consulting with your physician to rule out any other medical concern.
Regarding your niddah status, the key factor is the timing of staining relative to the timing of relations. As long as the staining is not discovered immediately after relations (within fifteen seconds), it is subject to the laws of stains (detailed in our articles on Stains and Toilet Paper). For example, it can be disregarded if found on a colored surface. Please see our page on Bleeding after Marital Relations for a more detailed discussion.
Going forward, we would recommend that you make sure to have relations on colored surfaces, to wait a bit between relations and cleaning yourself off, and for both you and your husband to clean yourselves with colored towels. Please see our page on Stains for more details.
To evaluate your current status based on past staining, we need more information about exactly what you saw (including size, shade and lighting conditions), at what interval after relations it was seen, where and how. For a quicker response, you may wish to contact our phone hotline.