Probably nothing (sorry, I ddin't mean to be flippant or rude). More likely, you're young and just haven't established a regular menstrual cycle yet. In other words, your hormonal cycle hasn't been set. This is quite common in teenagers and even in women in their 20s.
On the other hand, let's assume that you're more mature (?). A change in your menses could indicate that you're approaching menopause. Or perhaps that the lining of your uterus is either too thick or too thin. Either way, you tend to bleed, either sloughing away excess lining or from the rawness of the denuded uterine wall. Polyps & fibroids could also do this. An ultrasound would go a long ways towards explaining your 2 week period.
More important is how you feel. We'd be more concerned for someone who's lightheaded & dizzy from excess blood loss leading to anemia. So pay attention to your energy and what you feel as you change positions from laying down to sitting up and from sitting up to standing. If rapid changes in position make you feel lightheaded, you need to get evaluated for possible anemia from excess menstrual blood loss (and/or dehydration). Good luck!
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