Wa `alaykum as-salam wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Having a premarital relationship is definitely prohibited whether during Ramadan or after it. So, it is our advice, dear sister, to fear Allah, come back to Him, make use of this blessed month to change your lifestyle for better in a way that pleases Allah, and to have your sins forgiven.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states,
Fornication is a most grievous sin in Islam; the sin is even graver in the month of Ramadan. Ramadan is the month given to us to break free of all our sins and bad habits. Prophet Muhammad (peace and blessings be upon him) said, “When Ramadan arrives, the doors of Heaven are wide open and the doors of Hell are shut down, and the devils are chained, and an announcer will say, ‘O doer of good, come forward! O doer of bad, restrain yourself!'”
So, you are best advised to use this blessed month as a time to repent and change. This is a golden opportunity, and it could be the only one you have, for who knows whether Allah will give us another chance; we never know whether we will live to see another Ramadan. We see all around us people are being called back to Allah.
Death knows neither age nor status. Therefore, let us heed the warning of Allah’s Messenger, “Woe to those who witnessed Ramadan and yet failed to get their sins pardoned (by failing to utilize the month properly).”
If, however, you are married, then of course, you are allowed to have intimate spousal relations after breaking the fast. Allah says, “It is lawful for you to go in unto your wives during the night preceding the [day’s] fast: They are as a garment for you, and you are as a garment for them.” (Al-Baqarah 2:187)
Allah Almighty knows best.