When she was 13, she was lying down watching TV one day when she felt a lump in her abdomen. 'I thought it was food. It felt like a ball and I could move it, but I didn't think it was anything to worry about,' she says.
'But it stayed there for a few days, and that's when I told my mother. I went for tests and my white blood cell count was very high.'
At first doctors thought it might be cancer and conducted an exploratory operation. 'They knew they had to find out what this mass was. When they opened me up they found a lot of disease in there,' says Anastacia.
In fact, she had Crohn's disease, an inflammatory bowel condition that causes a range of distressing symptoms. She says: 'The doctors took out a mass the size of a grapefruit, then they took out lymph nodes, and some of the intestine. They sewed me back up with the scar I have on my tummy.'
lowers the waist-band of her denim skirt to reveal a 4in gash. 'I hated that scar for a long time,' she says. 'It was the only thing that bothered me about the disease - I thought they'd ruined my life. They'd given me a big scar and no guy would ever want to touch me and love me - or that was pretty much what I thought.'
Obsessed with this disfigurement, she otherwise ignored her medical condition, although the steroid drugs she took made her gain 21/2st. Then when she was 19, she embarked on a fruit and fibre diet after a producer told her she was too large.
'With my disease, that's like eating glass because it was all fibre. I had no idea what I was doing to myself. I hadn't had a problem with my stomach for years and I was sure the doctors had got it wrong.
'Then it hit me. I got the most terrible, excruciating pains in my stomach. My intestines collapsed and I had to be fed through a tube directly into one of the large veins in my chest.
'I was on a very high dose of liquid steroids, my hair got thin and my skin broke out. I gained a lot of weight immediately and I had a moon face.'
Not surprisingly she became depressed, until she looked at it in a different way. 'I learned not to be vain.
Your outer shell is not that important - your spirit and your heart mean the most,' she says.
She also learned to handle her disease. 'I decided I needed to face my illness. I began to understand what having it would mean for the rest of my life.
'Crohn's is an intestinal disorder and it is incurable. A bad attack can kill you. Your abdomen becomes inflamed and there are lots of nasty things - diarrhoea, vomiting, fevers, cramps and anal bleeding - to deal with.'
Anastacia now avoids anything that might spark off her Crohn's. 'I don't drink unless I've seen the schedule and know what is coming up work-wise - and even then I would only have a couple of glasses of wine.
'As for drugs, I would never do that. I'm a milk and cookies rock star and proud of it.'
Most importantly, she makes every effort to minimise stress because although Crohn's isn't caused by it, in her experience it is certainly affected by it.
'Bottling things up fuels the symptoms of Crohn's. I have had to learn not to be frightened of feelings. You might think it strange that I still wanted to fulfil my ambition of becoming a singer, trying to make it in a business that is so difficult and so pressurised,' she says.
'But to me that wasn't an issue, because I had changed the way I thought about things. I felt at ease with my emotions. I didn't let myself get wound up when I wasn't making it as a singer.
'I gave myself time and if I got rejected I allowed myself to get upset and then moved on, and continued with other work - as a waitress and a hairdresser. When I finally got offered a record deal two years ago, I was so happy I screamed.'
Doctors have told her that the disease could affect her fertility, but Anastacia has dealt with this in a characteristically positive manner.
'I can't guarantee that I will ever be able to have a child. If it comes down to it, I will adopt. That is so much bigger than anything you could ever achieve in the music business.'
Since her rise to fame, she has become friends with David and Victoria Beckham, and becomes effusive at the mention of their relationship.
'I look at them and think: "See, that's the way it should be.î You should have a family, you should work hard and you should love your partner. That's my fantasy.
'My illness has changed me in so many ways. It affects everything from my health to my job to my relationships. I recently broke up from a boyfriend, which was tough, but I'm OK.
'What is seen as a curse for some is a gift for me because it has helped me to discover who I really am.