Mel B was rushed to hospital on Friday after losing her sight.
The Spice Girls singer - who has no vision in her left eye after laser surgery went wrong - was rushed to London's Moorfield Eye Hospital after she was suddenly unable to see out of her right eye.
Mel arrived at the specialist facility wearing an eye patch and reportedly said: "I can't see anything I am totally blind."
A source added to The Sun on Sunday newspaper: "She was clearly in a lot of pain."
The 43-year-old star was later transferred to Luton and Dunstable Hospital's ophthalmology department and once her vision returned, she defied doctors orders to rejoin her bandmates at tour rehearsals.
Mel underwent "very swift and very good" care but will still require medication and further intensive treatment, likely to include more trips to hospital.
A source said: "Mel was in a really bad way. The other girls and their managers were really worried.
"Mel was frantic and screaming that she couldn't see a thing. She went to the hospital in a chauffeur-driven car with an assistant, who was holding her hand.
"No one knew what brought it on but she had very swift, and very good, treatment.
"It is unclear if the loss of vision is linked to her previous problems, although it does seem likely."
With the Spice Girls reunion tour due to kick off very soon, the 'Stop' hitmaker's bandmates, Geri Horner, Emma Bunton, and Mel C, are rallying around their friend and have promised to keep a close eye on her during the shows.
The insider added: "All the girls rallied round her, including Geri. They made sure they weren't behind on rehearsals, which have been going really well.
"Mel was determined to get back in action and was very brave.
"The situation did cause concern, and it is something the girls and those around them will monitor closely when the concerts start."