If you experienced a flow of blood after intercourse caused by further tearing of the hymen, you are rendered niddah, even though it was not the first act of intercourse after marriage. There is a special decree regarding hymenal bleeding that it is considered dam niddah and not dam makkah.
However, if you did not experience an actual flow, rather you found a stain of blood, your status depends on where the stain was found and how much time elapsed after intercourse before finding the stain. For example, if a stain was found on colored bedsheets, and 15 seconds elapsed after finishing intercourse before finding the stain, you are not rendered niddah. Please get back to us with more details of where the stain was found, its size and approximate color, and how soon after intercourse it was found.
Please note that there is no obligation to look for staining after intercourse, especially if there is concern about additional hymenal bleeding. You may take precautions against becoming niddah from any staining after intercourse by waiting a few minutes before getting up or cleaning yourselves. Use colored sheets. Be sure to use colored towels or disposable tissues/wipes to clean yourselves and do not look for any stains.
Finding blood after intercourse is a serious halachic concern. In order to attribute the blood found to the hymen (and thus eliminate this halachic concern), there should be other symptoms, such as pain or bleeding each time you have had intercourse since the wedding night. Another way of confirming the source is via visual examination by a doctor or bodeket taharah. If there is no clear confirmation of the source of the bleeding, the couple may be required to check for bleeding before and after intercourse on three occasions. If this is the case, please get back to us for further guidance.
Additionally, if you have experienced dam betulim (or pain) with each episode of intercourse since the wedding night, you may continue waiting the four day minimum instead of five before counting the clean days if you do become niddah from dam betulim. This leniency applies only to hymenal bleeding, not to menstrual bleeding.
Please feel free to get back to us with any further questions.