Spotting

So, I just started taking birth control, and this would be my third month of it. My first month, I forgot to take it one day, and I got terrible cramps and started lightly bleeding. It lasted for several days, and I still got a normal period when I was taking the sugar pills. Last month, I didn't miss any pills, but I started spotting around the same time as the first month, and it lasted as long as it had the first time, and I got a period again when taking the sugar pills. This month, I forgot one pill and the same thing happened: terrible cramps and spotting for several days. I'm a little confused. Am I having a mini-period between my periods? Is this normal?