Ok. Thanks for the info. The way that birth control pills work is, they contain a little estrogen and a little progesterone. These are hormones that your own ovaries make, and, left to their own devices, the levels of these hormones go up and down throughout the month. The pill, by contrast, gives you a small, consistent amount of hormone every day, as opposed to the natural ups and downs of your cycle. What this daily consistent amount of hormone does is suppress your ovaries from releasing an egg, and keep the inside lining of your uterus thin so that your periods become lighter, shorter, less crampy, and predictably occurring during your placebo week. (Here's some info from Planned Parenthood with info about how the pills work to prevent pregnancy.) Once you stop the pills for more than 7 days or so, your own natural hormone levels rise as your natural hormonal cycle attempts to resume itself. When you again add your birth control pills back to the mix, your uterus is now receiving mixed signals, as it is getting hormone from the pills while at the same time your natural hormones, which have been rising all week, are also trying to tell your uterus what to do. This can in turn cause some unexpected and irregular bleeding as the inside lining of your uterus grows and sheds in response to all these hormones.
I typically tell people that once you restart pills after taking a break from them, you can expect your cycles to be a bit irregular for the first pill pack or so. You also need to use a backup form of contraception (like condoms) until you've been back on the active pills for at least 2 weeks, because it takes 2 weeks of active pills for the pills to adequately suppress ovulation (the release of an egg from your ovary).
Good luck to you. Please leave a rating if feel so inclined.