Assalamu Alaykum.
Thank you for sharing such a personal, heartfelt experience with us. I can imagine how difficult it must have been to be in severe pain and longing for your mother's comfort.

Before we look into the meaning, it is wonderful to remember that the Prophet Muhammad (ﷺ) said, "Nothing is left of prophethood except glad tidings." People asked, "What are glad tidings?" He replied, "True good dreams." (Sahih al-Bukhari).

To answer your question about why you saw this before fully falling asleep: when we are in deep distress or physical pain, and our hearts sincerely yearn for someone we love, our souls can experience what is known as a true vision (Ru'ya) even in that delicate state between wakefulness and sleep. It is a moment where the veil is slightly lifted to grant you spiritual comfort.

Here is what this beautiful vision signifies based on classical Islamic interpretations:

Seeing your deceased mother sleeping peacefully is a wonderful sign of her resting in a state of peace and mercy in the Hereafter. The fact that you called out to her in your pain and immediately saw her shows a deep spiritual bond. She appeared to bring tranquility to your aching heart.

The arrival of the pregnant guest with a daughter and a son is a very strong symbol of approaching relief. In our tradition, a pregnant woman often symbolizes a burden or hardship that is about to come to an end, resulting in joy. The daughter represents worldly ease, happiness, and the opening of doors that felt closed, while the son signifies glad tidings and strength.

The fact that the lady was Christian and brought flowers suggests that goodness, healing, or unexpected joy will come to you from sources you might not anticipate. Flowers are universal symbols of pure joy, relief from sorrow, and beautiful, uplifting moments.

Finally, your mother waking up to warmly greet them is the most beautiful part. It signifies that the blessings, relief, and ease coming toward your home are welcomed and perhaps even tied to the sincere prayers (duas) your mother made for you during her lifetime.

Guidance for you: Take this vision as a gentle, divine reassurance. Your pain was felt, and this comforting scene was sent to soothe you. Whenever you remember this, make a heartfelt dua for your mother’s continued elevation in Jannah, and consider giving a small amount of charity (Sadaqah) on her behalf. Have hope that the physical pain or emotional heaviness you experienced is passing, making way for unexpected comfort and ease.

Please take care of your health, and may Allah grant you complete healing and peace of mind. Ameen!