I think you are looking at two separate matters here. Her attendance is one thing and I am inclined to agree with you that when a child is ill, they should definitely be allowed time to recuperate.
As for the reading and writing, I agree with the teacher that age doesn't have a lot to do with it. Every child is different, meaning that they all reach milestones at different ages. I can assure you that the teacher does not "expect" your daughter to be at the same level of other children just because they are in the same class. I'm assuming that your daughter is in stage/ability appropriate groups for reading and writing so I don't know how she can be classed by her teacher as behind in the first place. As for extra support, there is not a lot that the class teacher can do apart from maybe sending work home. It would not be deemed fair if she was to spend extra time with your daughter in class above other children who may also be struggling (which I am sure there will be a few). You say you read to her every night. That is excellent!! You should ask her teacher what method they use to teach reading and writing skills, whether it be JollyPhonics, Read/Write Inc. etc... This way you would be able to research it and continue the work at home along with the fabulous story time you already do. This will build her confidence in her own ability very quickly.
And i'm sorry if my response sounds harsh but