how can you get pregnant while on the pill?

I hear many can get pregnant while on the pills. I've been on the pills for an 1 year or more..so i was very normal and i thought my body adjusted to the pills already?! But, I'm worried if this is true. What are the symptoms if you continue having your mentrual cycles... Mine were different these past two months, last month it was shorter than usual. Now I'm bleeding.. the day after i was suppose to end. What is happening to my body. I've been feeling extremely dizzy @ times and nausea short of appetite.


Open to any advice...

Thanks