You have to remember how birth control pills work...They are actually the hormones your body procuces when you are pregnant, and they "trick" your body in to thinking you are pregnant, therefore, you don't release an egg that month.
That's why some women gain weight when they are on the pill. It's like your body is pregnant. Then when you take the placebos, your body is no longer receiving those hormones, and says, "okay, we aren't pregnant, let's release the blood from the uterus since it isn't needed for a fetus".
The pills you are taking now may be stronger or be a different combination of hormones, and that's why you feel the "symptoms" stronger than before.
The reason for the placebos with birth control pills is because it's easier for some women to stay on a regular schedule of taking a pill every night. For me, if I would have not taken anything for 7 days, I may not have remembered to start taking them when I was supposed to.
Libby