As the old saying goes, absence makes the heart grow fonder, and while Phatik's mother had earlier been eager to see him leave for Calcutta, siding with his brother Makhan without question when conflict rose between the two boys, her reaction when she comes to see Phatik is one of desperate worry.
Her reaction is that which you would expect of any mother with a sick child. Knowing that the boy is seriously ill, Bishamber has called for Phatik's mother, and she arrives in the room "like a whirlwind," which implies that she has come to her ailing son's bedside as fast as possible.
What's more, her distress is shown by the fact that she begins to "toss from side to side." This tells us that her distress is severe enough that she is unable to sit still. Over and above this, she is crying "in a loud voice," which tells us that she is immensely concerned for her son.
She refers to Phatik as her "darling," repeating the refrain as her son shows signs of waking up. She speaks to him with tones of affection, unlike before he had left for Calcutta.
She is described as having "flung" herself on the bed upon arrival, which implies a movement of desperation and panic.
In a nutshell, Phatik's mother reacts with love, fear, and desperation when she comes to see her sick son.