This has happened more than thrice last week. My menstruation has ended and my husband immediately asked to have coitus. However, I don’t remember whether I’ve taken my obligatory bath or not. What is the ruling for this?
Basically, intercourse isn’t permissible when the wife is menstruating (major hadas). Allah SWT states:
وَلَا تَقْرَبُوهُنَّ حَتَّىٰ يَطْهُرْنَ ۖ فَإِذَا تَطَهَّرْنَ فَأْتُوهُنَّ مِنْ حَيْثُ أَمَرَكُمُ اللَّـهُ
“So, keep away from wives during menstruation. And do not approach them until they are pure. And when they have purified themselves, then come to them from where Allah has ordained for you.” 
Ibn Kathir explained, “…Allah prohibited from approaching them (their wives) by having intercourse for as long as they are menstruating. It can be understood that: If the menstruation has mended, then it became permissible again.” 
According to Ibn Qudamah, the majority of scholars from madhhab Maliki, Syafie, Hanbal and others set the condition of first completing the obligatory bath after menstruation for it to be permissible for a woman to have intercourse with her husband.  When Ibn ‘Abbas commented on Surah al-Baqarah, verse 222, he set the condition that menstruation must have ended and she must have taken the obligatory bath before having intercourse.  The same is emphasized by our teacher Syeikh Dr Wahbah al-Zuhaili.
I’d like to advise you to confirm as to whether you’ve performed your obligatory bath or otherwise. If you have taken your obligatory bath, then there’s no issue in the matter. However, if you haven’t performed your obligatory bath or is unsure, then it returns to the original ruling which is you haven’t performed the obligatory bath.
اليَقِين لَا يُزَالُ بِالشَّكّ
“Certainty isn’t abrogated by doubt (syak).”
This is in accordance with the Islamic legal maxim:
الْأَصْل بَقَاء مَا كَانَ عَلَى مَا كَانَ
“The original ruling for something is that something remains according to its original ruling.”
Hence, if intercourse happened during menstruation, Imam Nawawi states that according to the opinion of madhhab al-Syafie, one isn’t fined or sentenced with kaffarah. However, he/she has to ask for forgiveness from Allah and repent for their actions. It is sunnah to donate one dinar if the intercourse happens at the beginning of her menstruation and half a dinar if it happens at the end of her menstruation. 
From Ibn Abbas, when the Prophet PBUH was mentioning intercourse during menstruation he said:
يَتَصَدَّقُ بِدِينَارٍ ، أَوْ بِنِصْفِ دِينَارٍ
“Let him give a Dinar or half a Dinar in charity.” 
It is best to avoid the khilaf and give in charity according to one’s capability to the poor and needy. I suggest and advise every husband and wife or couple to first learn the rulings in marriage before and after they have married, including the fiqh related to husband and wife. This is so that our every action will always follow the shariah and attain the blessing of Allah SWT. Wallahu a’lam.