How do you know if your birth control is working?

I am 25, just got back on birth control after a few years of being off. I've been having a lot of breakthrough bleeding this first month. Today I finished the last of my active pills for my first month and I had unprotected today. It says that you are protected after the first 7 days. I'm just worried because I'm hoping I began my pills at the right time. (My periods were very irregular). Has anyone else been nervous about this or am I just worrying over nothing?