I started spotting dark redish brown on a friday. Today is tuesday and I have been on birth control for a year and have never had this happen.?

Not uncommon. . This is not an uncommon symptom. I would stay on the current regimen and not miss any pills. If you feel well, continue on and you will most likely have return of normal menses if you have any doubts of a problem such as abdominal pain, pelvic pain, nausea, feeling of faintness or any worrisome problem, get seen immediately and get a pregnancy test. Speak with your prescriber, they need to no.
It can happen. First, make sure you're not pregnant. If you're not, the longer a woman is taking birth control pills the greater the chance her periods will change. If you're concerned, let your gyn know so you can be evaluated.