Today is the 16th of Dec. I get my period between the 19th and the 23rd of every month I started crampin very bad to were sometimes I can't walk yesterday and today they gotten worst I have bright ed bleeding but very light and very tender breast I know I can't be pregnant because the last time I had sex was a month ago and it was protect,so what does this mean?

It sounds like a regular period to me, with cramps. The color can vary from bright red, to dark red, to brown. It can be thin or clotted, light or heavy. Breasts are often tender.
Are the cramps something new?
Having regular cycles doesn't mean you won't have irregular ones too.
But having said all that, you CAN get pregnant using virtually any form of BC. And you can have a period after getting pregnant, usually a light one. You can also have a very early miscarriage.
It might put your mind at ease if you buy a test.