Hello! I`m 17 years old and I have had too short luteal phase ( about 7 - 8 days) three last cycles. Can it be caused by stress or underweight? Please help:(

The luteal phase is not typically not affected by factors like stress. For each individual women this phase is more consistent and will generally not vary more than a few days. It's the follicular (estrongenic) phase that can vary by many factors.
In order to know the difference between these 2 phases you must know your cycle very well by knowing when you ovulate (it is not typically on day 14 like most people think). Are you charting to know when your luteal phase actually begins?

At first thank you for answering:) I`m not charting, but I can feel when I`m ovulating. Before those last three cycles I was ovulating exactly 14 days before the next period:confused:

I would recommend going to Ovulation Calendar and Ovulation Chart - FertilityFriend.com (http://www.fertilityfriend.com) and it will teach you everything you need to know. It will give you a more accurate timing of when you are ovulating.

Thank you very much. It was very informative:)