There are many factors which may affect you menstrual flow, stresses and psychological factors are very important.
Since this is the first time and you usually have normal and regular periods, you need not to worry at all and just wait for the other period to come.
If the condition recurs may be we should have some laboratory investigations:
FSH, LH, TSH
and qualitative hcg
These tests should be done the second or third day after onset of menstrual blood flow.
At the end I reassure you that there is nothing to worry about and that menstrual irregularities are common at your age.
I hope these words were usefull to you and I wish you best luck.
Dr. Ahmed Bahaa.