What Causes A Light Menstrual Period While On Birth Control Pills?

This is my first month taking the pill. However during this month i have suffered from flu like illness with symptoms of D&V and fever. Ive been told this can affect the pill protecting me from getting pregnant also during this month i missed 3 days of them and i have been sexual active with my partner. I have come on my period however it was very very light almost watery and it only lasted two- three days i know the pill is meant to make my periods lighter but my normal cycle last 7days and are really heavy. Im now not sure if theres a chance if im pregnant?