My 13 year old daughter , started her periods in june for the first two month it lasted for 7 day but month it going on 3 weeks , is this ok or something to be worried out?

Early cycles: Young women often do not ovulate much the first year of menses but the estrogen driving the changes will bring on cycles. Irregular flow cycles occurring from 2 to 10 weeks are occasionally seen. The longer intervals may trigger heavier or longer flow. Once they begin to ovulate on a regular basis (6 mo-1 yr)a 2nd hormone stabilizes the pattern.

Dr Ungerleider agreed

Normal variation: Some girls have regular periods monthly right away, lasting 2-7 days, but it is very common to have irregular periods lasting longer or shorter than that. The flow amount can also vary a lot. This often happens for the first 2 years of having periods.