I wasn't sure whether to put this in children's or women's health but went for women's health as it's about periods. My 13 year old daughter got her periods last September. Her cycle has been really short (around 15 days) except for this last one, which was 24 days... but she's now been bleeding for 16 days. Initially it was getting lighter around day 5-7 as you'd expect, stayed light and then over the last couple of days has got as heavy as her period usually is.
She feels fine and isn't presenting with any hormonal symptoms but I'm not sure what to do. It's the opposite of the experience I had with my early periods, when I had long breaks between periods and then they were horrifically painful. Am very glad she's not experiencing that at least!