Likelihood of pregnancy

Hello, I have a question:

I have been on the birth control pill Loestrin for about three years, and have been on various birth control pills for about ten years without any breaks in between.
During the time of what would normally be a "fertile period," I had unprotected sex and forgot to take my pill. As soon as I remembered I took two, plus another one at my normal time the next day, meaning I took 3 pills in a 24 hour period. I have been taking my pills regularly since then (+ or - a few hours) and have not been using a backup method during sex. Is there a likelihood that I could have been impregnated? Also, under what circumstances does pregnancy on the pill occur (I hear it is possible) so I can be more cautious to avoid this in the future?

Thank you