Seaona Hamilton, 27, had complained about a sore head the night before to her mum but doctors still don't know what caused her sudden death.
The alarm was raised after she failed to pick up her two children - Honey, 5, and Ruairidh, 8, - from their after school club last Monday.
Her older sister Michelle, 31, and mum Janet rushed to her house and found her lying in the bathroom. An ambulance was called but there was nothing they could do.
Seona, from Clydebank, would have turned 28 on Sunday, three days before her son's 9th birthday.
Her mum told how she will be buried in the new dress she had planned to wear for her birthday night out with her close friends.
Janet, 49, said: "She had a onesie party with the kids the night before. I saw her and she told me her head felt like it was going to burst.
"I told her to go to the doctor in the morning. I wish I had thought more about it.
"She phoned the college in the morning. When I got home from work I got a phone call from the after- school club to say the children hadn't been picked up from school.
"I got a bit of a fright. I went down to the school and asked Honey and she said 'Mummy wasn't feeling well this morning.'
"When we got to her house, I could see her purse through the window. Her bathroom door was shut. I couldn't go in...my eldest daughter went in and found her.
"We called the ambulance but there was nothing they could do. There are still doing tests. She had her thyroid removed a few years ago, but they don't think it was anything to do with that.
"We just want to know what happened. Honey keeps asking 'What happened to my mummy?'
"We are all still in shock. Her sisters are keeping it together but I think they are staying strong for me.
"I saw her in the funeral parlour. Seaona loved her sleep and it looked like she was having a really good sleep.
"I saw her first and I will see her again today. I brought her into the world so I want to be the first and last person to see her."
Janet told how Seaona, a single mum who has another sister Emma, 25, had recently started a beauty therapy course at West College Scotland in Clydebank to "build a future" for her children, who are pupils at Edinbarnet Primary School in Faifley. She is a former pupil of Braidfield High School.
Janet said: "She was a fun mum. The kids absolutely adored her. She was so funny and full of life. She used to get on to me about her name because it was such an unusual spelling.
"I can't believe the number of people who have left messages on Facebook. She had a big circle of friends and was very close to her sisters.
"We are a very close family. She would come into this house and then leave and it would be so quiet. It will be quiet forever now."
Stephanie Graham, vice principal of West College Scotland paid tribute to an "outstanding student".
She said: "Seaona was an extremely popular, committed and talented young woman.
"She was an outstanding student and a role model to other students in the department."
The funeral will take place tomorrow at 12.30pm at Faifley Parish Church followed by a service at Clydebank Crematorium in North Dalnottar. The family have asked that mourners wear bright colours to honour her personality.
Janet, who works for Marks and Spencer and is married to Seaona's step-dad Andy, 57, has vowed to take care of her two grandchildren. She said: "I know that's what she would have wanted."