Like you, I have just started using cilest, and had heard some pretty awful things about it too! But I'm nearing the end of my first packet and have to admit that things have been great on it, and I'm feeling great too! I was nauseous on day 2, but put that down to having a major exam that day, and things have been fine since! I get the occasional headache, but that's nothing new for me. Have had some slight spotting, but nothing too major.
In response to your question, your period will continue as normal, it won't stop. Then you will have your withdrawal bleed after your 21 days of taking the pill, and start your new packet after the 7 day break (on the same day that you took the pill in the first place).
Hope that makes sense! If you have any more questions etc, then ask away