Am i pregnant? do i have infection?pls help me out..thanks a lot!!!?

I usually had a regular period. i got my first day of period april 11, and I took mercilon pills on that day..my boyfirend and i had a night together april 16...and we never had it again...it was the first and the last...since day one of my period up to now may 2 I'm experiencing breakthrough bleeding or spotting..the last time i took mercilon was 2 yrs ago since then i never took mercilon due to inactive life...the last day I took mercilon pills was yesterday may 1...do i have the chance of getting pregnant? do i have an infection that's why I'm experiencing bleeding or it was just an effect of the ?... the bleeding is enough for me to use napkin..the bleeding varies from light to medium flow bleeding...I don't feel any cramps or pain...it bothers me a lot..do i have to visit a doctor or do i have to observe the bleeding for a few more weeks?