Christina Aguilera was ordered to cancel two of her world tour dates after collapsing backstage.The 'Dirrty' singer, who is reported to be four months pregnant, suffered breathing difficulties during a performance in Melbourne, Australia, last Friday. Although she managed to finish the concert, the star was forced to miss some songs and her encore. Doctors were called to the venue and after treating Christina they ordered her to cancel her two remaining Melbourne dates and confined her to bed rest.Christina - who has yet to confirm the pregnancy herself - apologised to her fans, saying she was suffering from flu. She said: "I want to personally apologise to all of my fans."I am truly disappointed that I won't be able to share my show with you all."The 'Beautiful' singer is set to fly to New Zealand today to finish the last two dates of the 'Back To Basics' tour. However, friends are concerned she is over-doing it and want her to slowdown. One said: "She is a perfectionist and hates letting people down. But we're hoping she will cancel the last two dates and return to Los Angeles to put her feet up."Earlier this month, Christina's estranged father, Fausto, confirmed his daughter was expecting her first child with husband Jordan Bratman. He said: "I'm so excited. I want Christina to achieve all her goals. I wish her all the happiness in the world."