Although you didn't provide a lot of details ("sick" can mean a lot of different things), there are some things you can do.
Start by keeping a daily log of her activities. Keep a record (detailed) of what/when she eats, her activities, time she wakes up/naps/goes to bed, electronics usage (for example, note if she is on her computer in bed at bedtime), and most of all, her complaints and symptoms. Note if she is having a coughing fit, if she vomits, if she says she feels weak, if she has a fever, if she can't go to school and why, what's going on at school the days she misses - you get the idea. Keep one page per day in a paper notebook, and keep track of dates and times.
See if there are any patterns.
How does she eat? What are her activities?
Does the doctor simply say "chest infection" and send her on her way with more antibiotics? What antibiotics has she been on? Do you feel like this doctor is really paying attention to you and your daughter?
Is the doctor giving you other advice that perhaps your daughter is not complying with? H
How long has this been going on?
Have you asked the pharmacist about side effects of these antibiotics, or how to properly take them, or if your daughter should be taking a certain vitamin or supplement when she takes the antibiotics? Does she take probiotics?
If you can provide more information, someone might have an idea about how to help you.