Stains found immediately after intercourse render a woman niddah. Further, there are additional halachic problems with finding blood immediately after intercourse. If you did wipe yourself immediately after relations, please get back to us for further guidance.
However, if some time passed (you waited a few minutes after intercourse) before wiping yourself with a tissue, then the stain does not render you niddah. We do suggest you abstain from relations for 24 hours after the staining subsides until your status is clarified.
In the future we suggest that you wait a few minutes after relations before wiping yourself, and to use a tissue or a colored towel. You should avoid looking for any stains. You should also take precautions against becoming niddah from staining as outlined in our article on stains.
Please feel free to get back to us with any further questions.