My 11yr old has had her period for 16 days straight. its very light and has been very light the whole time. she just started her period a few months ago. is this normal? her doctor said its periods to fluctuate when they are in this first year?

During the first-year menses can be irregular in frequency and flow. Egg release is inconsistent, and that function creates part of the hormonal balance of regularity. I would let her know this should normalize over time, manage as needed and get her on a daily iron containing vitamin. When she goes in for her next checkup discuss it with her doc or do so sooner if you wish.