Hello
Thanks for your query at DoctorSpring.com
At times, you might have a scantier period than usual, don't worry.
If the bleeding was very less, just take a pregnancy test to be sure.
However, once in a while, such light bleeding could occur.
Your menstrual cycle gets affected by - stress, sleep, exercise, weight changes, travelling, worrying, smoking, drinking, late nights, hormonal medications etc.
If this happens each month, then you need to consult an OB - GYN and have an internal evaluation, Pap smear and a pelvic ultrasound scan.
All the best
Please feel free to discuss further.

Yes there is a chance of pregnancy if you are sexually active, but better get tested for confirmation
Regards