My sister has only had her period for about a yr. now and they are very unpredictable. Sometimes she will go several months without having one. Then it will be really light to medium for 3-4 days, always at different times each month. Lately her periods have gotten heavier and this month she has had her period twice during two different weeks, with a week in between of nothing. She didn't have her period in Feb., but should she be concerned that she had 2 periods in 1 month? Is that normal or ok?
Hope, The first year can be VERY irregular. This is quite normal. She might not even be ovulating every cycle. How old is she and is her health and body weight ok? Is the rest of her sexual development happening as it should?
If you are very worried about it, get her to a doctor. Also encourage her to keep an accurate calender of her periods. It is very useful to have an accurate record when you talk to a doctor about it.